Merge remote-tracking branch 'upstream/master' into master

1 files changed, 595 insertions, 379 deletions

diff --git a/python.html.markdown b/python.html.markdown
index 0cc33a80..2fc266eb 100644
--- a/python.html.markdown
+++ b/python.html.markdown
@@ -1,32 +1,22 @@
 ---
-language: python
+language: Python
 contributors:
-    - ["Louie Dinh", "http://ldinh.ca"]
-    - ["Amin Bandali", "https://aminb.org"]
+    - ["Louie Dinh", "http://pythonpracticeprojects.com"]
+    - ["Steven Basart", "http://github.com/xksteven"]
     - ["Andre Polykanine", "https://github.com/Oire"]
+    - ["Zachary Ferguson", "http://github.com/zfergus2"]
     - ["evuez", "http://github.com/evuez"]
-    - ["asyne", "https://github.com/justblah"]
-    - ["habi", "http://github.com/habi"]
     - ["Rommel Martinez", "https://ebzzry.io"]
+    - ["Roberto Fernandez Diaz", "https://github.com/robertofd1995"]
+    - ["caminsha", "https://github.com/caminsha"]
 filename: learnpython.py
 ---
 
-Python was created by Guido Van Rossum in the early 90s. It is now one of the
-most popular languages in existence. I fell in love with Python for its
-syntactic clarity. It's basically executable pseudocode.
+Python was created by Guido van Rossum in the early 90s. It is now one of the most popular
+languages in existence. I fell in love with Python for its syntactic clarity. It's basically
+executable pseudocode.
 
-Feedback would be highly appreciated! You can reach me at [@louiedinh](http://twitter.com/louiedinh)
-or louiedinh [at] [google's email service]
-
-Note: This article applies to Python 2.7 specifically, but should be applicable
-to Python 2.x. Python 2.7 is reaching end of life and will stop being
-maintained in 2020, it is though recommended to start learning Python with
-Python 3. For Python 3.x, take a look at the [Python 3 tutorial](http://learnxinyminutes.com/docs/python3/).
-
-It is also possible to write Python code which is compatible with Python 2.7
-and 3.x at the same time, using Python [`__future__` imports](https://docs.python.org/2/library/__future__.html). `__future__` imports
-allow you to write Python 3 code that will run on Python 2, so check out the
-Python 3 tutorial.
+Note: This article applies to Python 3 specifically. Check out [here](http://learnxinyminutes.com/docs/pythonlegacy/) if you want to learn the old Python 2.7
 
 ```python
 
@@ -34,67 +24,72 @@ Python 3 tutorial.
 
 """ Multiline strings can be written
     using three "s, and are often used
-    as comments
+    as documentation.
 """
 
 ####################################################
-# 1. Primitive Datatypes and Operators
+## 1. Primitive Datatypes and Operators
 ####################################################
 
 # You have numbers
 3  # => 3
 
 # Math is what you would expect
-1 + 1  # => 2
-8 - 1  # => 7
+1 + 1   # => 2
+8 - 1   # => 7
 10 * 2  # => 20
-35 / 5  # => 7
-
-# Division is a bit tricky. It is integer division and floors the results
-# automatically.
-5 / 2  # => 2
+35 / 5  # => 7.0
 
-# To fix division we need to learn about floats.
-2.0  # This is a float
-11.0 / 4.0  # => 2.75 ahhh...much better
-
-# Result of integer division truncated down both for positive and negative.
-5 // 3  # => 1
-5.0 // 3.0  # => 1.0 # works on floats too
--5 // 3  # => -2
+# Integer division rounds down for both positive and negative numbers.
+5 // 3       # => 1
+-5 // 3      # => -2
+5.0 // 3.0   # => 1.0 # works on floats too
 -5.0 // 3.0  # => -2.0
 
-# Note that we can also import division module(Section 6 Modules)
-# to carry out normal division with just one '/'.
-from __future__ import division
-
-11 / 4  # => 2.75  ...normal division
-11 // 4  # => 2 ...floored division
+# The result of division is always a float
+10.0 / 3  # => 3.3333333333333335
 
 # Modulo operation
 7 % 3  # => 1
 
-# Exponentiation (x to the yth power)
-2 ** 4  # => 16
+# Exponentiation (x**y, x to the yth power)
+2**3  # => 8
 
 # Enforce precedence with parentheses
+1 + 3 * 2  # => 7
 (1 + 3) * 2  # => 8
 
+# Boolean values are primitives (Note: the capitalization)
+True  # => True
+False  # => False
+
+# negate with not
+not True   # => False
+not False  # => True
+
 # Boolean Operators
 # Note "and" and "or" are case-sensitive
 True and False  # => False
-False or True  # => True
+False or True   # => True
 
-# Note using Bool operators with ints
-0 and 2  # => 0
--5 or 0  # => -5
-0 == False  # => True
-2 == True  # => False
-1 == True  # => True
+# True and False are actually 1 and 0 but with different keywords
+True + True # => 2
+True * 8    # => 8
+False - 5   # => -5
 
-# negate with not
-not True  # => False
-not False  # => True
+# Comparison operators look at the numerical value of True and False
+0 == False  # => True
+1 == True   # => True
+2 == True   # => False
+-5 != False # => True
+
+# Using boolean logical operators on ints casts them to booleans for evaluation, but their non-cast value is returned
+# Don't mix up with bool(ints) and bitwise and/or (&,|)
+bool(0)     # => False
+bool(4)     # => True
+bool(-6)    # => True
+0 and 2     # => 0
+-5 or 0     # => -5
 
 # Equality is ==
 1 == 1  # => True
@@ -110,93 +105,88 @@ not False  # => True
 2 <= 2  # => True
 2 >= 2  # => True
 
-# Comparisons can be chained!
+# Seeing whether a value is in a range
+1 < 2 and 2 < 3  # => True
+2 < 3 and 3 < 2  # => False
+# Chaining makes this look nicer
 1 < 2 < 3  # => True
 2 < 3 < 2  # => False
 
+# (is vs. ==) is checks if two variables refer to the same object, but == checks
+# if the objects pointed to have the same values.
+a = [1, 2, 3, 4]  # Point a at a new list, [1, 2, 3, 4]
+b = a             # Point b at what a is pointing to
+b is a            # => True, a and b refer to the same object
+b == a            # => True, a's and b's objects are equal
+b = [1, 2, 3, 4]  # Point b at a new list, [1, 2, 3, 4]
+b is a            # => False, a and b do not refer to the same object
+b == a            # => True, a's and b's objects are equal
+
 # Strings are created with " or '
 "This is a string."
 'This is also a string.'
 
-# Strings can be added too!
+# Strings can be added too! But try not to do this.
 "Hello " + "world!"  # => "Hello world!"
-# Strings can be added without using '+'
-"Hello " "world!"  # => "Hello world!"
-
-# ... or multiplied
-"Hello" * 3  # => "HelloHelloHello"
+# String literals (but not variables) can be concatenated without using '+'
+"Hello " "world!"    # => "Hello world!"
 
 # A string can be treated like a list of characters
-"This is a string"[0]  # => 'T'
+"Hello world!"[0]  # => 'H'
 
 # You can find the length of a string
 len("This is a string")  # => 16
 
-# String formatting with %
-# Even though the % string operator will be deprecated on Python 3.1 and removed
-# later at some time, it may still be good to know how it works.
-x = 'apple'
-y = 'lemon'
-z = "The items in the basket are %s and %s" % (x, y)
+# You can also format using f-strings or formatted string literals (in Python 3.6+)
+name = "Reiko"
+f"She said her name is {name}." # => "She said her name is Reiko"
+# You can basically put any Python statement inside the braces and it will be output in the string.
+f"{name} is {len(name)} characters long." # => "Reiko is 5 characters long."
 
-# A newer way to format strings is the format method.
-# This method is the preferred way
-"{} is a {}".format("This", "placeholder")
-"{0} can be {1}".format("strings", "formatted")
-# You can use keywords if you don't want to count.
-"{name} wants to eat {food}".format(name="Bob", food="lasagna")
 
 # None is an object
 None  # => None
 
 # Don't use the equality "==" symbol to compare objects to None
-# Use "is" instead
+# Use "is" instead. This checks for equality of object identity.
 "etc" is None  # => False
-None is None  # => True
-
-# The 'is' operator tests for object identity. This isn't
-# very useful when dealing with primitive values, but is
-# very useful when dealing with objects.
-
-# Any object can be used in a Boolean context.
-# The following values are considered falsey:
-#    - None
-#    - zero of any numeric type (e.g., 0, 0L, 0.0, 0j)
-#    - empty sequences (e.g., '', (), [])
-#    - empty containers (e.g., {}, set())
-#    - instances of user-defined classes meeting certain conditions
-#      see: https://docs.python.org/2/reference/datamodel.html#object.__nonzero__
-#
-# All other values are truthy (using the bool() function on them returns True).
-bool(0)  # => False
-bool("")  # => False
+None is None   # => True
 
+# None, 0, and empty strings/lists/dicts/tuples all evaluate to False.
+# All other values are True
+bool(0)   # => False
+bool("")  # => False
+bool([])  # => False
+bool({})  # => False
+bool(())  # => False
 
 ####################################################
-# 2. Variables and Collections
+## 2. Variables and Collections
 ####################################################
 
-# Python has a print statement
-print "I'm Python. Nice to meet you!"  # => I'm Python. Nice to meet you!
+# Python has a print function
+print("I'm Python. Nice to meet you!")  # => I'm Python. Nice to meet you!
+
+# By default the print function also prints out a newline at the end.
+# Use the optional argument end to change the end string.
+print("Hello, World", end="!")  # => Hello, World!
 
 # Simple way to get input data from console
-input_string_var = raw_input(
-    "Enter some data: ")  # Returns the data as a string
-input_var = input("Enter some data: ")  # Evaluates the data as python code
-# Warning: Caution is recommended for input() method usage
-# Note: In python 3, input() is deprecated and raw_input() is renamed to input()
-
-# No need to declare variables before assigning to them.
-some_var = 5  # Convention is to use lower_case_with_underscores
+input_string_var = input("Enter some data: ") # Returns the data as a string
+# Note: In earlier versions of Python, input() method was named as raw_input()
+
+# There are no declarations, only assignments.
+# Convention is to use lower_case_with_underscores
+some_var = 5
 some_var  # => 5
 
 # Accessing a previously unassigned variable is an exception.
 # See Control Flow to learn more about exception handling.
-some_other_var  # Raises a name error
+some_unknown_var  # Raises a NameError
 
 # if can be used as an expression
 # Equivalent of C's '?:' ternary operator
-"yahoo!" if 3 > 2 else 2  # => "yahoo!"
+"yay!" if 0 > 1 else "nay!"  # => "nay!"
 
 # Lists store sequences
 li = []
@@ -204,21 +194,17 @@ li = []
 other_li = [4, 5, 6]
 
 # Add stuff to the end of a list with append
-li.append(1)  # li is now [1]
-li.append(2)  # li is now [1, 2]
-li.append(4)  # li is now [1, 2, 4]
-li.append(3)  # li is now [1, 2, 4, 3]
+li.append(1)    # li is now [1]
+li.append(2)    # li is now [1, 2]
+li.append(4)    # li is now [1, 2, 4]
+li.append(3)    # li is now [1, 2, 4, 3]
 # Remove from the end with pop
-li.pop()  # => 3 and li is now [1, 2, 4]
+li.pop()        # => 3 and li is now [1, 2, 4]
 # Let's put it back
-li.append(3)  # li is now [1, 2, 4, 3] again.
+li.append(3)    # li is now [1, 2, 4, 3] again.
 
 # Access a list like you would any array
-li[0]  # => 1
-# Assign new values to indexes that have already been initialized with =
-li[0] = 42
-li[0]  # => 42
-li[0] = 1  # Note: setting it back to the original value
+li[0]   # => 1
 # Look at the last element
 li[-1]  # => 3
 
@@ -226,39 +212,39 @@ li[-1]  # => 3
 li[4]  # Raises an IndexError
 
 # You can look at ranges with slice syntax.
+# The start index is included, the end index is not
 # (It's a closed/open range for you mathy types.)
-li[1:3]  # => [2, 4]
-# Omit the beginning
-li[2:]  # => [4, 3]
-# Omit the end
-li[:3]  # => [1, 2, 4]
-# Select every second entry
-li[::2]  # =>[1, 4]
-# Reverse a copy of the list
-li[::-1]  # => [3, 4, 2, 1]
+li[1:3]   # Return list from index 1 to 3 => [2, 4]
+li[2:]    # Return list starting from index 2 => [4, 3]
+li[:3]    # Return list from beginning until index 3  => [1, 2, 4]
+li[::2]   # Return list selecting every second entry => [1, 4]
+li[::-1]  # Return list in reverse order => [3, 4, 2, 1]
 # Use any combination of these to make advanced slices
 # li[start:end:step]
 
+# Make a one layer deep copy using slices
+li2 = li[:]  # => li2 = [1, 2, 4, 3] but (li2 is li) will result in false.
+
 # Remove arbitrary elements from a list with "del"
 del li[2]  # li is now [1, 2, 3]
 
-# You can add lists
-li + other_li  # => [1, 2, 3, 4, 5, 6]
-# Note: values for li and for other_li are not modified.
-
-# Concatenate lists with "extend()"
-li.extend(other_li)  # Now li is [1, 2, 3, 4, 5, 6]
-
 # Remove first occurrence of a value
-li.remove(2)  # li is now [1, 3, 4, 5, 6]
+li.remove(2)  # li is now [1, 3]
 li.remove(2)  # Raises a ValueError as 2 is not in the list
 
 # Insert an element at a specific index
-li.insert(1, 2)  # li is now [1, 2, 3, 4, 5, 6] again
+li.insert(1, 2)  # li is now [1, 2, 3] again
 
-# Get the index of the first item found
+# Get the index of the first item found matching the argument
 li.index(2)  # => 1
-li.index(7)  # Raises a ValueError as 7 is not in the list
+li.index(4)  # Raises a ValueError as 4 is not in the list
+
+# You can add lists
+# Note: values for li and for other_li are not modified.
+li + other_li  # => [1, 2, 3, 4, 5, 6]
+
+# Concatenate lists with "extend()"
+li.extend(other_li)  # Now li is [1, 2, 3, 4, 5, 6]
 
 # Check for existence in a list with "in"
 1 in li  # => True
@@ -266,82 +252,109 @@ li.index(7)  # Raises a ValueError as 7 is not in the list
 # Examine the length with "len()"
 len(li)  # => 6
 
+
 # Tuples are like lists but are immutable.
 tup = (1, 2, 3)
-tup[0]  # => 1
+tup[0]      # => 1
 tup[0] = 3  # Raises a TypeError
 
-# You can do all those list thingies on tuples too
-len(tup)  # => 3
+# Note that a tuple of length one has to have a comma after the last element but
+# tuples of other lengths, even zero, do not.
+type((1))   # => <class 'int'>
+type((1,))  # => <class 'tuple'>
+type(())    # => <class 'tuple'>
+
+# You can do most of the list operations on tuples too
+len(tup)         # => 3
 tup + (4, 5, 6)  # => (1, 2, 3, 4, 5, 6)
-tup[:2]  # => (1, 2)
-2 in tup  # => True
+tup[:2]          # => (1, 2)
+2 in tup         # => True
 
 # You can unpack tuples (or lists) into variables
 a, b, c = (1, 2, 3)  # a is now 1, b is now 2 and c is now 3
-d, e, f = 4, 5, 6  # you can leave out the parentheses
+# You can also do extended unpacking
+a, *b, c = (1, 2, 3, 4)  # a is now 1, b is now [2, 3] and c is now 4
 # Tuples are created by default if you leave out the parentheses
-g = 4, 5, 6  # => (4, 5, 6)
+d, e, f = 4, 5, 6  # tuple 4, 5, 6 is unpacked into variables d, e and f
+# respectively such that d = 4, e = 5 and f = 6
 # Now look how easy it is to swap two values
 e, d = d, e  # d is now 5 and e is now 4
 
-# Dictionaries store mappings
+
+# Dictionaries store mappings from keys to values
 empty_dict = {}
 # Here is a prefilled dictionary
 filled_dict = {"one": 1, "two": 2, "three": 3}
 
+# Note keys for dictionaries have to be immutable types. This is to ensure that
+# the key can be converted to a constant hash value for quick look-ups.
+# Immutable types include ints, floats, strings, tuples.
+invalid_dict = {[1,2,3]: "123"}  # => Raises a TypeError: unhashable type: 'list'
+valid_dict = {(1,2,3):[1,2,3]}   # Values can be of any type, however.
+
 # Look up values with []
 filled_dict["one"]  # => 1
 
-# Get all keys as a list with "keys()"
-filled_dict.keys()  # => ["three", "two", "one"]
-# Note - Dictionary key ordering is not guaranteed.
-# Your results might not match this exactly.
+# Get all keys as an iterable with "keys()". We need to wrap the call in list()
+# to turn it into a list. We'll talk about those later.  Note - for Python
+# versions <3.7, dictionary key ordering is not guaranteed. Your results might
+# not match the example below exactly. However, as of Python 3.7, dictionary
+# items maintain the order at which they are inserted into the dictionary.
+list(filled_dict.keys())  # => ["three", "two", "one"] in Python <3.7
+list(filled_dict.keys())  # => ["one", "two", "three"] in Python 3.7+
 
-# Get all values as a list with "values()"
-filled_dict.values()  # => [3, 2, 1]
-# Note - Same as above regarding key ordering.
 
-# Get all key-value pairs as a list of tuples with "items()"
-filled_dict.items()  # => [("one", 1), ("two", 2), ("three", 3)]
+# Get all values as an iterable with "values()". Once again we need to wrap it
+# in list() to get it out of the iterable. Note - Same as above regarding key
+# ordering.
+list(filled_dict.values())  # => [3, 2, 1]  in Python <3.7
+list(filled_dict.values())  # => [1, 2, 3] in Python 3.7+
 
 # Check for existence of keys in a dictionary with "in"
 "one" in filled_dict  # => True
-1 in filled_dict  # => False
+1 in filled_dict      # => False
 
 # Looking up a non-existing key is a KeyError
 filled_dict["four"]  # KeyError
 
 # Use "get()" method to avoid the KeyError
-filled_dict.get("one")  # => 1
-filled_dict.get("four")  # => None
+filled_dict.get("one")      # => 1
+filled_dict.get("four")     # => None
 # The get method supports a default argument when the value is missing
-filled_dict.get("one", 4)  # => 1
+filled_dict.get("one", 4)   # => 1
 filled_dict.get("four", 4)  # => 4
-# note that filled_dict.get("four") is still => None
-# (get doesn't set the value in the dictionary)
-
-# set the value of a key with a syntax similar to lists
-filled_dict["four"] = 4  # now, filled_dict["four"] => 4
 
 # "setdefault()" inserts into a dictionary only if the given key isn't present
 filled_dict.setdefault("five", 5)  # filled_dict["five"] is set to 5
 filled_dict.setdefault("five", 6)  # filled_dict["five"] is still 5
 
-# You can declare sets (which are like unordered lists that cannot contain
-# duplicate values) using the set object.
-empty_set = set()
-# Initialize a "set()" with a bunch of values
-some_set = set([1, 2, 2, 3, 4])  # some_set is now set([1, 2, 3, 4])
+# Adding to a dictionary
+filled_dict.update({"four":4})  # => {"one": 1, "two": 2, "three": 3, "four": 4}
+filled_dict["four"] = 4         # another way to add to dict
+
+# Remove keys from a dictionary with del
+del filled_dict["one"]  # Removes the key "one" from filled dict
+
+# From Python 3.5 you can also use the additional unpacking options
+{'a': 1, **{'b': 2}}  # => {'a': 1, 'b': 2}
+{'a': 1, **{'a': 2}}  # => {'a': 2}
+
+
 
-# order is not guaranteed, even though it may sometimes look sorted
-another_set = set([4, 3, 2, 2, 1])  # another_set is now set([1, 2, 3, 4])
+# Sets store ... well sets
+empty_set = set()
+# Initialize a set with a bunch of values. Yeah, it looks a bit like a dict. Sorry.
+some_set = {1, 1, 2, 2, 3, 4}  # some_set is now {1, 2, 3, 4}
 
-# Since Python 2.7, {} can be used to declare a set
-filled_set = {1, 2, 2, 3, 4}  # => {1, 2, 3, 4}
+# Similar to keys of a dictionary, elements of a set have to be immutable.
+invalid_set = {[1], 1}  # => Raises a TypeError: unhashable type: 'list'
+valid_set = {(1,), 1}
 
-# Add more items to a set
+# Add one more item to the set
+filled_set = some_set
 filled_set.add(5)  # filled_set is now {1, 2, 3, 4, 5}
+# Sets do not have duplicate elements
+filled_set.add(5)  # it remains as before {1, 2, 3, 4, 5}
 
 # Do set intersection with &
 other_set = {3, 4, 5, 6}
@@ -357,37 +370,37 @@ filled_set | other_set  # => {1, 2, 3, 4, 5, 6}
 {1, 2, 3, 4} ^ {2, 3, 5}  # => {1, 4, 5}
 
 # Check if set on the left is a superset of set on the right
-{1, 2} >= {1, 2, 3}  # => False
+{1, 2} >= {1, 2, 3} # => False
 
 # Check if set on the left is a subset of set on the right
-{1, 2} <= {1, 2, 3}  # => True
+{1, 2} <= {1, 2, 3} # => True
 
 # Check for existence in a set with in
-2 in filled_set  # => True
+2 in filled_set   # => True
 10 in filled_set  # => False
-10 not in filled_set # => True
 
-# Check data type of variable
-type(li)   # => list
-type(filled_dict)   # => dict
-type(5)   # => int
+# Make a one layer deep copy
+filled_set = some_set.copy()  # filled_set is {1, 2, 3, 4, 5}
+filled_set is some_set        # => False
 
 
 ####################################################
-#  3. Control Flow
+## 3. Control Flow and Iterables
 ####################################################
 
 # Let's just make a variable
 some_var = 5
 
-# Here is an if statement. Indentation is significant in python!
-# prints "some_var is smaller than 10"
+# Here is an if statement. Indentation is significant in Python!
+# Convention is to use four spaces, not tabs.
+# This prints "some_var is smaller than 10"
 if some_var > 10:
-    print "some_var is totally bigger than 10."
-elif some_var < 10:  # This elif clause is optional.
-    print "some_var is smaller than 10."
-else:  # This is optional too.
-    print "some_var is indeed 10."
+    print("some_var is totally bigger than 10.")
+elif some_var < 10:    # This elif clause is optional.
+    print("some_var is smaller than 10.")
+else:                  # This is optional too.
+    print("some_var is indeed 10.")
+
 
 """
 For loops iterate over lists
@@ -397,11 +410,11 @@ prints:
     mouse is a mammal
 """
 for animal in ["dog", "cat", "mouse"]:
-    # You can use {0} to interpolate formatted strings. (See above.)
-    print "{0} is a mammal".format(animal)
+    # You can use format() to interpolate formatted strings
+    print("{} is a mammal".format(animal))
 
 """
-"range(number)" returns a list of numbers
+"range(number)" returns an iterable of numbers
 from zero to the given number
 prints:
     0
@@ -410,10 +423,10 @@ prints:
     3
 """
 for i in range(4):
-    print i
+    print(i)
 
 """
-"range(lower, upper)" returns a list of numbers
+"range(lower, upper)" returns an iterable of numbers
 from the lower number to the upper number
 prints:
     4
@@ -422,7 +435,29 @@ prints:
     7
 """
 for i in range(4, 8):
-    print i
+    print(i)
+
+"""
+"range(lower, upper, step)" returns an iterable of numbers
+from the lower number to the upper number, while incrementing
+by step. If step is not indicated, the default value is 1.
+prints:
+    4
+    6
+"""
+for i in range(4, 8, 2):
+    print(i)
+
+"""
+To loop over a list, and retrieve both the index and the value of each item in the list
+prints:
+    0 dog
+    1 cat
+    2 mouse
+"""
+animals = ["dog", "cat", "mouse"]
+for i, value in enumerate(animals):
+    print(i, value)
 
 """
 While loops go until a condition is no longer met.
@@ -434,72 +469,121 @@ prints:
 """
 x = 0
 while x < 4:
-    print x
+    print(x)
     x += 1  # Shorthand for x = x + 1
 
 # Handle exceptions with a try/except block
-
-# Works on Python 2.6 and up:
 try:
     # Use "raise" to raise an error
     raise IndexError("This is an index error")
 except IndexError as e:
-    pass  # Pass is just a no-op. Usually you would do recovery here.
+    pass                 # Pass is just a no-op. Usually you would do recovery here.
 except (TypeError, NameError):
-    pass  # Multiple exceptions can be handled together, if required.
-else:  # Optional clause to the try/except block. Must follow all except blocks
-    print "All good!"  # Runs only if the code in try raises no exceptions
-finally:  # Execute under all circumstances
-    print "We can clean up resources here"
+    pass                 # Multiple exceptions can be handled together, if required.
+else:                    # Optional clause to the try/except block. Must follow all except blocks
+    print("All good!")   # Runs only if the code in try raises no exceptions
+finally:                 #  Execute under all circumstances
+    print("We can clean up resources here")
 
 # Instead of try/finally to cleanup resources you can use a with statement
 with open("myfile.txt") as f:
     for line in f:
-        print line
+        print(line)
+
+# Writing to a file
+contents = {"aa": 12, "bb": 21}
+with open("myfile1.txt", "w+") as file:
+    file.write(str(contents))        # writes a string to a file
+
+with open("myfile2.txt", "w+") as file:
+    file.write(json.dumps(contents)) # writes an object to a file
+
+# Reading from a file
+with open('myfile1.txt', "r+") as file:
+    contents = file.read()           # reads a string from a file
+print(contents)
+# print: {"aa": 12, "bb": 21}
+
+with open('myfile2.txt', "r+") as file:
+    contents = json.load(file)       # reads a json object from a file
+print(contents)     
+# print: {"aa": 12, "bb": 21}
+
+
+# Python offers a fundamental abstraction called the Iterable.
+# An iterable is an object that can be treated as a sequence.
+# The object returned by the range function, is an iterable.
+
+filled_dict = {"one": 1, "two": 2, "three": 3}
+our_iterable = filled_dict.keys()
+print(our_iterable)  # => dict_keys(['one', 'two', 'three']). This is an object that implements our Iterable interface.
+
+# We can loop over it.
+for i in our_iterable:
+    print(i)  # Prints one, two, three
+
+# However we cannot address elements by index.
+our_iterable[1]  # Raises a TypeError
+
+# An iterable is an object that knows how to create an iterator.
+our_iterator = iter(our_iterable)
+
+# Our iterator is an object that can remember the state as we traverse through it.
+# We get the next object with "next()".
+next(our_iterator)  # => "one"
+
+# It maintains state as we iterate.
+next(our_iterator)  # => "two"
+next(our_iterator)  # => "three"
+
+# After the iterator has returned all of its data, it raises a StopIteration exception
+next(our_iterator)  # Raises StopIteration
+
+# We can also loop over it, in fact, "for" does this implicitly!
+our_iterator = iter(our_iterable)
+for i in our_iterator:
+    print(i)  # Prints one, two, three
+
+# You can grab all the elements of an iterable or iterator by calling list() on it.
+list(our_iterable)  # => Returns ["one", "two", "three"]
+list(our_iterator)  # => Returns [] because state is saved
 
 
 ####################################################
-# 4. Functions
+## 4. Functions
 ####################################################
 
 # Use "def" to create new functions
 def add(x, y):
-    print "x is {0} and y is {1}".format(x, y)
+    print("x is {} and y is {}".format(x, y))
     return x + y  # Return values with a return statement
 
-
 # Calling functions with parameters
 add(5, 6)  # => prints out "x is 5 and y is 6" and returns 11
 
 # Another way to call functions is with keyword arguments
 add(y=6, x=5)  # Keyword arguments can arrive in any order.
 
-
 # You can define functions that take a variable number of
-# positional args, which will be interpreted as a tuple by using *
+# positional arguments
 def varargs(*args):
     return args
 
-
 varargs(1, 2, 3)  # => (1, 2, 3)
 
-
 # You can define functions that take a variable number of
-# keyword args, as well, which will be interpreted as a dict by using **
+# keyword arguments, as well
 def keyword_args(**kwargs):
     return kwargs
 
-
 # Let's call it to see what happens
 keyword_args(big="foot", loch="ness")  # => {"big": "foot", "loch": "ness"}
 
 
 # You can do both at once, if you like
 def all_the_args(*args, **kwargs):
-    print args
-    print kwargs
-
-
+    print(args)
+    print(kwargs)
 """
 all_the_args(1, 2, a=3, b=4) prints:
     (1, 2)
@@ -507,38 +591,36 @@ all_the_args(1, 2, a=3, b=4) prints:
 """
 
 # When calling functions, you can do the opposite of args/kwargs!
-# Use * to expand positional args and use ** to expand keyword args.
+# Use * to expand tuples and use ** to expand kwargs.
 args = (1, 2, 3, 4)
 kwargs = {"a": 3, "b": 4}
-all_the_args(*args)  # equivalent to all_the_args(1, 2, 3, 4)
-all_the_args(**kwargs)  # equivalent to all_the_args(a=3, b=4)
+all_the_args(*args)            # equivalent to all_the_args(1, 2, 3, 4)
+all_the_args(**kwargs)         # equivalent to all_the_args(a=3, b=4)
 all_the_args(*args, **kwargs)  # equivalent to all_the_args(1, 2, 3, 4, a=3, b=4)
 
+# Returning multiple values (with tuple assignments)
+def swap(x, y):
+    return y, x  # Return multiple values as a tuple without the parenthesis.
+                 # (Note: parenthesis have been excluded but can be included)
 
-# you can pass args and kwargs along to other functions that take args/kwargs
-# by expanding them with * and ** respectively
-def pass_all_the_args(*args, **kwargs):
-    all_the_args(*args, **kwargs)
-    print varargs(*args)
-    print keyword_args(**kwargs)
-
+x = 1
+y = 2
+x, y = swap(x, y)     # => x = 2, y = 1
+# (x, y) = swap(x,y)  # Again parenthesis have been excluded but can be included.
 
 # Function Scope
 x = 5
 
-
 def set_x(num):
     # Local var x not the same as global variable x
-    x = num  # => 43
-    print x  # => 43
-
+    x = num    # => 43
+    print(x)   # => 43
 
 def set_global_x(num):
     global x
-    print x  # => 5
-    x = num  # global var x is now set to 6
-    print x  # => 6
-
+    print(x)   # => 5
+    x = num    # global var x is now set to 6
+    print(x)   # => 6
 
 set_x(43)
 set_global_x(6)
@@ -548,55 +630,98 @@ set_global_x(6)
 def create_adder(x):
     def adder(y):
         return x + y
-
     return adder
 
-
 add_10 = create_adder(10)
-add_10(3)  # => 13
+add_10(3)   # => 13
 
 # There are also anonymous functions
-(lambda x: x > 2)(3)  # => True
+(lambda x: x > 2)(3)                  # => True
 (lambda x, y: x ** 2 + y ** 2)(2, 1)  # => 5
 
 # There are built-in higher order functions
-map(add_10, [1, 2, 3])  # => [11, 12, 13]
-map(max, [1, 2, 3], [4, 2, 1])  # => [4, 2, 3]
+list(map(add_10, [1, 2, 3]))          # => [11, 12, 13]
+list(map(max, [1, 2, 3], [4, 2, 1]))  # => [4, 2, 3]
 
-filter(lambda x: x > 5, [3, 4, 5, 6, 7])  # => [6, 7]
+list(filter(lambda x: x > 5, [3, 4, 5, 6, 7]))  # => [6, 7]
 
 # We can use list comprehensions for nice maps and filters
-[add_10(i) for i in [1, 2, 3]]  # => [11, 12, 13]
+# List comprehension stores the output as a list which can itself be a nested list
+[add_10(i) for i in [1, 2, 3]]         # => [11, 12, 13]
 [x for x in [3, 4, 5, 6, 7] if x > 5]  # => [6, 7]
 
 # You can construct set and dict comprehensions as well.
-{x for x in 'abcddeef' if x in 'abc'}  # => {'a', 'b', 'c'}
-{x: x ** 2 for x in range(5)}  # => {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
+{x for x in 'abcddeef' if x not in 'abc'}  # => {'d', 'e', 'f'}
+{x: x**2 for x in range(5)}  # => {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}
 
 
 ####################################################
-# 5. Classes
+## 5. Modules
 ####################################################
 
-# We subclass from object to get a class.
-class Human(object):
+# You can import modules
+import math
+print(math.sqrt(16))  # => 4.0
+
+# You can get specific functions from a module
+from math import ceil, floor
+print(ceil(3.7))   # => 4.0
+print(floor(3.7))  # => 3.0
+
+# You can import all functions from a module.
+# Warning: this is not recommended
+from math import *
+
+# You can shorten module names
+import math as m
+math.sqrt(16) == m.sqrt(16)  # => True
+
+# Python modules are just ordinary Python files. You
+# can write your own, and import them. The name of the
+# module is the same as the name of the file.
+
+# You can find out which functions and attributes
+# are defined in a module.
+import math
+dir(math)
+
+# If you have a Python script named math.py in the same
+# folder as your current script, the file math.py will
+# be loaded instead of the built-in Python module.
+# This happens because the local folder has priority
+# over Python's built-in libraries.
+
+
+####################################################
+## 6. Classes
+####################################################
+
+# We use the "class" statement to create a class
+class Human:
+
     # A class attribute. It is shared by all instances of this class
     species = "H. sapiens"
 
     # Basic initializer, this is called when this class is instantiated.
     # Note that the double leading and trailing underscores denote objects
-    # or attributes that are used by python but that live in user-controlled
-    # namespaces. You should not invent such names on your own.
+    # or attributes that are used by Python but that live in user-controlled
+    # namespaces. Methods(or objects or attributes) like: __init__, __str__,
+    # __repr__ etc. are called special methods (or sometimes called dunder methods)
+    # You should not invent such names on your own.
     def __init__(self, name):
         # Assign the argument to the instance's name attribute
         self.name = name
 
         # Initialize property
-        self.age = 0
+        self._age = 0
 
     # An instance method. All methods take "self" as the first argument
     def say(self, msg):
-        return "{0}: {1}".format(self.name, msg)
+        print("{name}: {message}".format(name=self.name, message=msg))
+
+    # Another instance method
+    def sing(self):
+        return 'yo... yo... microphone check... one two... one two...'
 
     # A class method is shared among all instances
     # They are called with the calling class as the first argument
@@ -610,8 +735,8 @@ class Human(object):
         return "*grunt*"
 
     # A property is just like a getter.
-    # It turns the method age() into an read-only attribute
-    # of the same name.
+    # It turns the method age() into an read-only attribute of the same name.
+    # There's no need to write trivial getters and setters in Python, though.
     @property
     def age(self):
         return self._age
@@ -627,160 +752,253 @@ class Human(object):
         del self._age
 
 
-# Instantiate a class
-i = Human(name="Ian")
-print i.say("hi")  # prints out "Ian: hi"
+# When a Python interpreter reads a source file it executes all its code.
+# This __name__ check makes sure this code block is only executed when this
+# module is the main program.
+if __name__ == '__main__':
+    # Instantiate a class
+    i = Human(name="Ian")
+    i.say("hi")                     # "Ian: hi"
+    j = Human("Joel")
+    j.say("hello")                  # "Joel: hello"
+    # i and j are instances of type Human, or in other words: they are Human objects
+
+    # Call our class method
+    i.say(i.get_species())          # "Ian: H. sapiens"
+    # Change the shared attribute
+    Human.species = "H. neanderthalensis"
+    i.say(i.get_species())          # => "Ian: H. neanderthalensis"
+    j.say(j.get_species())          # => "Joel: H. neanderthalensis"
+
+    # Call the static method
+    print(Human.grunt())            # => "*grunt*"
+
+    # Static methods can be called by instances too
+    print(i.grunt())                # => "*grunt*"
+
+    # Update the property for this instance
+    i.age = 42
+    # Get the property
+    i.say(i.age)                    # => "Ian: 42"
+    j.say(j.age)                    # => "Joel: 0"
+    # Delete the property
+    del i.age
+    # i.age                         # => this would raise an AttributeError
 
-j = Human("Joel")
-print j.say("hello")  # prints out "Joel: hello"
 
-# Call our class method
-i.get_species()  # => "H. sapiens"
+####################################################
+## 6.1 Inheritance
+####################################################
 
-# Change the shared attribute
-Human.species = "H. neanderthalensis"
-i.get_species()  # => "H. neanderthalensis"
-j.get_species()  # => "H. neanderthalensis"
+# Inheritance allows new child classes to be defined that inherit methods and
+# variables from their parent class.
 
-# Call the static method
-Human.grunt()  # => "*grunt*"
+# Using the Human class defined above as the base or parent class, we can
+# define a child class, Superhero, which inherits the class variables like
+# "species", "name", and "age", as well as methods, like "sing" and "grunt"
+# from the Human class, but can also have its own unique properties.
 
-# Update the property
-i.age = 42
+# To take advantage of modularization by file you could place the classes above in their own files,
+# say, human.py
 
-# Get the property
-i.age  # => 42
+# To import functions from other files use the following format
+# from "filename-without-extension" import "function-or-class"
 
-# Delete the property
-del i.age
-i.age  # => raises an AttributeError
+from human import Human
 
-####################################################
-# 6. Modules
-####################################################
 
-# You can import modules
-import math
+# Specify the parent class(es) as parameters to the class definition
+class Superhero(Human):
 
-print math.sqrt(16)  # => 4.0
+    # If the child class should inherit all of the parent's definitions without
+    # any modifications, you can just use the "pass" keyword (and nothing else)
+    # but in this case it is commented out to allow for a unique child class:
+    # pass
 
-# You can get specific functions from a module
-from math import ceil, floor
+    # Child classes can override their parents' attributes
+    species = 'Superhuman'
 
-print ceil(3.7)  # => 4.0
-print floor(3.7)  # => 3.0
+    # Children automatically inherit their parent class's constructor including
+    # its arguments, but can also define additional arguments or definitions
+    # and override its methods such as the class constructor.
+    # This constructor inherits the "name" argument from the "Human" class and
+    # adds the "superpower" and "movie" arguments:
+    def __init__(self, name, movie=False,
+                 superpowers=["super strength", "bulletproofing"]):
 
-# You can import all functions from a module.
-# Warning: this is not recommended
-from math import *
+        # add additional class attributes:
+        self.fictional = True
+        self.movie = movie
+        # be aware of mutable default values, since defaults are shared
+        self.superpowers = superpowers
 
-# You can shorten module names
-import math as m
+        # The "super" function lets you access the parent class's methods
+        # that are overridden by the child, in this case, the __init__ method.
+        # This calls the parent class constructor:
+        super().__init__(name)
 
-math.sqrt(16) == m.sqrt(16)  # => True
-# you can also test that the functions are equivalent
-from math import sqrt
+    # override the sing method
+    def sing(self):
+        return 'Dun, dun, DUN!'
 
-math.sqrt == m.sqrt == sqrt  # => True
+    # add an additional instance method
+    def boast(self):
+        for power in self.superpowers:
+            print("I wield the power of {pow}!".format(pow=power))
 
-# Python modules are just ordinary python files. You
-# can write your own, and import them. The name of the
-# module is the same as the name of the file.
 
-# You can find out which functions and attributes
-# defines a module.
-import math
+if __name__ == '__main__':
+    sup = Superhero(name="Tick")
 
-dir(math)
+    # Instance type checks
+    if isinstance(sup, Human):
+        print('I am human')
+    if type(sup) is Superhero:
+        print('I am a superhero')
 
+    # Get the Method Resolution search Order used by both getattr() and super()
+    # This attribute is dynamic and can be updated
+    print(Superhero.__mro__)    # => (<class '__main__.Superhero'>,
+                                # => <class 'human.Human'>, <class 'object'>)
 
-# If you have a Python script named math.py in the same
-# folder as your current script, the file math.py will
-# be loaded instead of the built-in Python module.
-# This happens because the local folder has priority
-# over Python's built-in libraries.
+    # Calls parent method but uses its own class attribute
+    print(sup.get_species())    # => Superhuman
+
+    # Calls overridden method
+    print(sup.sing())           # => Dun, dun, DUN!
+
+    # Calls method from Human
+    sup.say('Spoon')            # => Tick: Spoon
 
+    # Call method that exists only in Superhero
+    sup.boast()                 # => I wield the power of super strength!
+                                # => I wield the power of bulletproofing!
+
+    # Inherited class attribute
+    sup.age = 31
+    print(sup.age)              # => 31
+
+    # Attribute that only exists within Superhero
+    print('Am I Oscar eligible? ' + str(sup.movie))
 
 ####################################################
-# 7. Advanced
+## 6.2 Multiple Inheritance
 ####################################################
 
-# Generators
-# A generator "generates" values as they are requested instead of storing
-# everything up front
+# Another class definition
+# bat.py
+class Bat:
 
-# The following method (*NOT* a generator) will double all values and store it
-# in `double_arr`. For large size of iterables, that might get huge!
-def double_numbers(iterable):
-    double_arr = []
-    for i in iterable:
-        double_arr.append(i + i)
-    return double_arr
+    species = 'Baty'
 
+    def __init__(self, can_fly=True):
+        self.fly = can_fly
 
-# Running the following would mean we'll double all values first and return all
-# of them back to be checked by our condition
-for value in double_numbers(range(1000000)):  # `test_non_generator`
-    print value
-    if value > 5:
-        break
+    # This class also has a say method
+    def say(self, msg):
+        msg = '... ... ...'
+        return msg
+
+    # And its own method as well
+    def sonar(self):
+        return '))) ... ((('
+
+if __name__ == '__main__':
+    b = Bat()
+    print(b.say('hello'))
+    print(b.fly)
+
+
+# And yet another class definition that inherits from Superhero and Bat
+# superhero.py
+from superhero import Superhero
+from bat import Bat
 
+# Define Batman as a child that inherits from both Superhero and Bat
+class Batman(Superhero, Bat):
 
-# We could instead use a generator to "generate" the doubled value as the item
-# is being requested
-def double_numbers_generator(iterable):
+    def __init__(self, *args, **kwargs):
+        # Typically to inherit attributes you have to call super:
+        # super(Batman, self).__init__(*args, **kwargs)      
+        # However we are dealing with multiple inheritance here, and super()
+        # only works with the next base class in the MRO list.
+        # So instead we explicitly call __init__ for all ancestors.
+        # The use of *args and **kwargs allows for a clean way to pass arguments,
+        # with each parent "peeling a layer of the onion".
+        Superhero.__init__(self, 'anonymous', movie=True,
+                           superpowers=['Wealthy'], *args, **kwargs)
+        Bat.__init__(self, *args, can_fly=False, **kwargs)
+        # override the value for the name attribute
+        self.name = 'Sad Affleck'
+
+    def sing(self):
+        return 'nan nan nan nan nan batman!'
+
+
+if __name__ == '__main__':
+    sup = Batman()
+
+    # Get the Method Resolution search Order used by both getattr() and super().
+    # This attribute is dynamic and can be updated
+    print(Batman.__mro__)       # => (<class '__main__.Batman'>,
+                                # => <class 'superhero.Superhero'>,
+                                # => <class 'human.Human'>,
+                                # => <class 'bat.Bat'>, <class 'object'>)
+
+    # Calls parent method but uses its own class attribute
+    print(sup.get_species())    # => Superhuman
+
+    # Calls overridden method
+    print(sup.sing())           # => nan nan nan nan nan batman!
+
+    # Calls method from Human, because inheritance order matters
+    sup.say('I agree')          # => Sad Affleck: I agree
+
+    # Call method that exists only in 2nd ancestor
+    print(sup.sonar())          # => ))) ... (((
+
+    # Inherited class attribute
+    sup.age = 100
+    print(sup.age)              # => 100
+
+    # Inherited attribute from 2nd ancestor whose default value was overridden.
+    print('Can I fly? ' + str(sup.fly)) # => Can I fly? False
+
+
+
+####################################################
+## 7. Advanced
+####################################################
+
+# Generators help you make lazy code.
+def double_numbers(iterable):
     for i in iterable:
         yield i + i
 
-
-# Running the same code as before, but with a generator, now allows us to iterate
-# over the values and doubling them one by one as they are being consumed by
-# our logic. Hence as soon as we see a value > 5, we break out of the
-# loop and don't need to double most of the values sent in (MUCH FASTER!)
-for value in double_numbers_generator(xrange(1000000)):  # `test_generator`
-    print value
-    if value > 5:
+# Generators are memory-efficient because they only load the data needed to
+# process the next value in the iterable. This allows them to perform
+# operations on otherwise prohibitively large value ranges.
+# NOTE: `range` replaces `xrange` in Python 3.
+for i in double_numbers(range(1, 900000000)):  # `range` is a generator.
+    print(i)
+    if i >= 30:
         break
 
-# BTW: did you notice the use of `range` in `test_non_generator` and `xrange` in `test_generator`?
-# Just as `double_numbers_generator` is the generator version of `double_numbers`
-# We have `xrange` as the generator version of `range`
-# `range` would return back and array with 1000000 values for us to use
-# `xrange` would generate 1000000 values for us as we request / iterate over those items
-
 # Just as you can create a list comprehension, you can create generator
 # comprehensions as well.
-values = (-x for x in [1, 2, 3, 4, 5])
+values = (-x for x in [1,2,3,4,5])
 for x in values:
     print(x)  # prints -1 -2 -3 -4 -5 to console/terminal
 
 # You can also cast a generator comprehension directly to a list.
-values = (-x for x in [1, 2, 3, 4, 5])
+values = (-x for x in [1,2,3,4,5])
 gen_to_list = list(values)
 print(gen_to_list)  # => [-1, -2, -3, -4, -5]
 
+
 # Decorators
-# A decorator is a higher order function, which accepts and returns a function.
-# Simple usage example – add_apples decorator will add 'Apple' element into
-# fruits list returned by get_fruits target function.
-def add_apples(func):
-    def get_fruits():
-        fruits = func()
-        fruits.append('Apple')
-        return fruits
-    return get_fruits
-
-@add_apples
-def get_fruits():
-    return ['Banana', 'Mango', 'Orange']
-
-# Prints out the list of fruits with 'Apple' element in it:
-# Banana, Mango, Orange, Apple
-print ', '.join(get_fruits())
-
-# in this example beg wraps say
-# Beg will call say. If say_please is True then it will change the returned
-# message
+# In this example `beg` wraps `say`. If say_please is True then it
+# will change the returned message.
 from functools import wraps
 
 
@@ -801,8 +1019,8 @@ def say(say_please=False):
     return msg, say_please
 
 
-print say()  # Can you buy me a beer?
-print say(say_please=True)  # Can you buy me a beer? Please! I am poor :(
+print(say())                 # Can you buy me a beer?
+print(say(say_please=True))  # Can you buy me a beer? Please! I am poor :(
 ```
 
 ## Ready For More?
@@ -810,18 +1028,16 @@ print say(say_please=True)  # Can you buy me a beer? Please! I am poor :(
 ### Free Online
 
 * [Automate the Boring Stuff with Python](https://automatetheboringstuff.com)
-* [Learn Python The Hard Way](http://learnpythonthehardway.org/book/)
-* [Dive Into Python](http://www.diveintopython.net/)
-* [The Official Docs](http://docs.python.org/2/)
+* [Ideas for Python Projects](http://pythonpracticeprojects.com)
+* [The Official Docs](http://docs.python.org/3/)
 * [Hitchhiker's Guide to Python](http://docs.python-guide.org/en/latest/)
-* [Python Module of the Week](http://pymotw.com/2/)
-* [A Crash Course in Python for Scientists](http://nbviewer.ipython.org/5920182)
+* [Python Course](http://www.python-course.eu/index.php)
 * [First Steps With Python](https://realpython.com/learn/python-first-steps/)
-* [LearnPython](http://www.learnpython.org/)
-* [Fullstack Python](https://www.fullstackpython.com/)
-
-### Dead Tree
-
-* [Programming Python](http://www.amazon.com/gp/product/0596158106/ref=as_li_qf_sp_asin_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0596158106&linkCode=as2&tag=homebits04-20)
-* [Dive Into Python](http://www.amazon.com/gp/product/1441413022/ref=as_li_tf_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=1441413022&linkCode=as2&tag=homebits04-20)
-* [Python Essential Reference](http://www.amazon.com/gp/product/0672329786/ref=as_li_tf_tl?ie=UTF8&camp=1789&creative=9325&creativeASIN=0672329786&linkCode=as2&tag=homebits04-20)
+* [A curated list of awesome Python frameworks, libraries and software](https://github.com/vinta/awesome-python)
+* [30 Python Language Features and Tricks You May Not Know About](http://sahandsaba.com/thirty-python-language-features-and-tricks-you-may-not-know.html)
+* [Official Style Guide for Python](https://www.python.org/dev/peps/pep-0008/)
+* [Python 3 Computer Science Circles](http://cscircles.cemc.uwaterloo.ca/)
+* [Dive Into Python 3](http://www.diveintopython3.net/index.html)
+* [A Crash Course in Python for Scientists](http://nbviewer.jupyter.org/gist/anonymous/5924718)
+* [Python Tutorial for Intermediates](https://pythonbasics.org/)
+* [Build a Desktop App with Python](https://pythonpyqt.com/)