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+---
+language: python3
+contributors:
+ - ["Louie Dinh", "http://pythonpracticeprojects.com"]
+ - ["Steven Basart", "http://github.com/xksteven"]
+ - ["Andre Polykanine", "https://github.com/Oire"]
+ - ["Zachary Ferguson", "http://github.com/zfergus2"]
+filename: learnpython3.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.
+
+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 3 specifically. Check out [here](http://learnxinyminutes.com/docs/python/) if you want to learn the old Python 2.7
+
+```python
+
+# Single line comments start with a number symbol.
+
+""" Multiline strings can be written
+ using three "s, and are often used
+ as comments
+"""
+
+####################################################
+## 1. Primitive Datatypes and Operators
+####################################################
+
+# You have numbers
+3 # => 3
+
+# Math is what you would expect
+1 + 1 # => 2
+8 - 1 # => 7
+10 * 2 # => 20
+
+# Except division which returns floats, real numbers, by default
+35 / 5 # => 7.0
+
+# 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
+-5.0 // 3.0 # => -2.0
+
+# When you use a float, results are floats
+3 * 2.0 # => 6.0
+
+# Modulo operation
+7 % 3 # => 1
+
+# Exponentiation (x**y, x to the yth power)
+2**4 # => 16
+
+# Enforce precedence with parentheses
+(1 + 3) * 2 # => 8
+
+# Boolean values are primitives (Note: the capitalization)
+True
+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
+
+# Note using Bool operators with ints
+0 and 2 #=> 0
+-5 or 0 #=> -5
+0 == False #=> True
+2 == True #=> False
+1 == True #=> True
+
+# Equality is ==
+1 == 1 # => True
+2 == 1 # => False
+
+# Inequality is !=
+1 != 1 # => False
+2 != 1 # => True
+
+# More comparisons
+1 < 10 # => True
+1 > 10 # => False
+2 <= 2 # => True
+2 >= 2 # => True
+
+# Comparisons can be chained!
+1 < 2 < 3 # => True
+2 < 3 < 2 # => False
+
+# (is vs. ==) is checks if two variable 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 a 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! But try not to do this.
+"Hello " + "world!" # => "Hello world!"
+# Strings can be added without using '+'
+"Hello " "world!" # => "Hello world!"
+
+# A string can be treated like a list of characters
+"This is a string"[0] # => 'T'
+
+# .format can be used to format strings, like this:
+"{} can be {}".format("strings", "interpolated")
+
+# You can repeat the formatting arguments to save some typing.
+"{0} be nimble, {0} be quick, {0} jump over the {1}".format("Jack", "candle stick")
+#=> "Jack be nimble, Jack be quick, Jack jump over the candle stick"
+
+# You can use keywords if you don't want to count.
+"{name} wants to eat {food}".format(name="Bob", food="lasagna") #=> "Bob wants to eat lasagna"
+
+# If your Python 3 code also needs to run on Python 2.5 and below, you can also
+# still use the old style of formatting:
+"%s can be %s the %s way" % ("strings", "interpolated", "old")
+
+
+# None is an object
+None # => None
+
+# Don't use the equality "==" symbol to compare objects to None
+# Use "is" instead. This checks for equality of object identity.
+"etc" is None # => False
+None is None # => True
+
+# None, 0, and empty strings/lists/dicts all evaluate to False.
+# All other values are True
+bool(0) # => False
+bool("") # => False
+bool([]) #=> False
+bool({}) #=> False
+
+
+####################################################
+## 2. Variables and Collections
+####################################################
+
+# Python has a print function
+print("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 character.
+print("Hello, World", end="!") # => Hello, World!
+
+# No need to declare variables before assigning to them.
+# 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_unknown_var # Raises a NameError
+
+# Lists store sequences
+li = []
+# You can start with a prefilled list
+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]
+# Remove from the end with pop
+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.
+
+# Access a list like you would any array
+li[0] # => 1
+# Look at the last element
+li[-1] # => 3
+
+# Looking out of bounds is an IndexError
+li[4] # Raises an IndexError
+
+# You can look at ranges with slice syntax.
+# (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]
+# Return a reversed copy of the list
+li[::-1] # => [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
+# 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
+
+# Examine the length with "len()"
+len(li) # => 6
+
+
+# Tuples are like lists but are immutable.
+tup = (1, 2, 3)
+tup[0] # => 1
+tup[0] = 3 # Raises a TypeError
+
+# 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
+
+# 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
+# Tuples are created by default if you leave out the parentheses
+d, e, f = 4, 5, 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
+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 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 - Dictionary key
+# ordering is not guaranteed. Your results might not match this exactly.
+list(filled_dict.keys()) # => ["three", "two", "one"]
+
+
+# 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]
+
+
+# Check for existence of keys in a dictionary with "in"
+"one" in filled_dict # => True
+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
+# The get method supports a default argument when the value is missing
+filled_dict.get("one", 4) # => 1
+filled_dict.get("four", 4) # => 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
+
+# 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
+
+
+# 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}
+
+# 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}
+
+# Can set new variables to a set
+filled_set = some_set
+
+# Add one more item to the set
+filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5}
+
+# Do set intersection with &
+other_set = {3, 4, 5, 6}
+filled_set & other_set # => {3, 4, 5}
+
+# Do set union with |
+filled_set | other_set # => {1, 2, 3, 4, 5, 6}
+
+# Do set difference with -
+{1, 2, 3, 4} - {2, 3, 5} # => {1, 4}
+
+# Check for existence in a set with in
+2 in filled_set # => True
+10 in filled_set # => False
+
+
+
+####################################################
+## 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"
+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.")
+
+
+"""
+For loops iterate over lists
+prints:
+ dog is a mammal
+ cat is a mammal
+ mouse is a mammal
+"""
+for animal in ["dog", "cat", "mouse"]:
+ # You can use format() to interpolate formatted strings
+ print("{} is a mammal".format(animal))
+
+"""
+"range(number)" returns an iterable of numbers
+from zero to the given number
+prints:
+ 0
+ 1
+ 2
+ 3
+"""
+for i in range(4):
+ print(i)
+
+"""
+"range(lower, upper)" returns an iterable of numbers
+from the lower number to the upper number
+prints:
+ 4
+ 5
+ 6
+ 7
+"""
+for i in range(4, 8):
+ 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
+ 8
+"""
+for i in range(4, 8, 2):
+ print(i)
+"""
+
+While loops go until a condition is no longer met.
+prints:
+ 0
+ 1
+ 2
+ 3
+"""
+x = 0
+while x < 4:
+ print(x)
+ x += 1 # Shorthand for x = x + 1
+
+# Handle exceptions with a try/except block
+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.
+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")
+
+# 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)
+
+# Python offers a fundamental abstraction called the Iterable.
+# An iterable is an object that can be treated as a sequence.
+# The object returned the range function, is an iterable.
+
+filled_dict = {"one": 1, "two": 2, "three": 3}
+our_iterable = filled_dict.keys()
+print(our_iterable) #=> range(1,10). 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 gives you a StopIterator Exception
+next(our_iterator) # Raises StopIteration
+
+# You can grab all the elements of an iterator by calling list() on it.
+list(filled_dict.keys()) #=> Returns ["one", "two", "three"]
+
+
+####################################################
+## 4. Functions
+####################################################
+
+# Use "def" to create new functions
+def add(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 arguments
+def varargs(*args):
+ return args
+
+varargs(1, 2, 3) # => (1, 2, 3)
+
+# You can define functions that take a variable number of
+# 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)
+"""
+all_the_args(1, 2, a=3, b=4) prints:
+ (1, 2)
+ {"a": 3, "b": 4}
+"""
+
+# When calling functions, you can do the opposite of args/kwargs!
+# 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 foo(1, 2, 3, 4)
+all_the_args(**kwargs) # equivalent to foo(a=3, b=4)
+all_the_args(*args, **kwargs) # equivalent to foo(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)
+
+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 setX(num):
+ # Local var x not the same as global variable x
+ x = num # => 43
+ print (x) # => 43
+
+def setGlobalX(num):
+ global x
+ print (x) # => 5
+ x = num # global var x is now set to 6
+ print (x) # => 6
+
+setX(43)
+setGlobalX(6)
+
+
+# Python has first class functions
+def create_adder(x):
+ def adder(y):
+ return x + y
+ return adder
+
+add_10 = create_adder(10)
+add_10(3) # => 13
+
+# There are also anonymous functions
+(lambda x: x > 2)(3) # => True
+
+# TODO - Fix for iterables
+# There are built-in higher order functions
+map(add_10, [1, 2, 3]) # => [11, 12, 13]
+filter(lambda x: x > 5, [3, 4, 5, 6, 7]) # => [6, 7]
+
+# We can use list comprehensions for nice maps and filters
+# 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]
+
+####################################################
+## 5. Classes
+####################################################
+
+
+# We subclass from object to get a class.
+class Human(object):
+
+ # 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. Methods(or objects or attributes) like: __init__, __str__,
+ # __repr__ etc. are called magic 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
+
+ # An instance method. All methods take "self" as the first argument
+ def say(self, msg):
+ return "{name}: {message}".format(name=self.name, message=msg)
+
+ # A class method is shared among all instances
+ # They are called with the calling class as the first argument
+ @classmethod
+ def get_species(cls):
+ return cls.species
+
+ # A static method is called without a class or instance reference
+ @staticmethod
+ def grunt():
+ return "*grunt*"
+
+
+# Instantiate a class
+i = Human(name="Ian")
+print(i.say("hi")) # prints out "Ian: hi"
+
+j = Human("Joel")
+print(j.say("hello")) # prints out "Joel: hello"
+
+# Call our class method
+i.get_species() # => "H. sapiens"
+
+# Change the shared attribute
+Human.species = "H. neanderthalensis"
+i.get_species() # => "H. neanderthalensis"
+j.get_species() # => "H. neanderthalensis"
+
+# Call the static method
+Human.grunt() # => "*grunt*"
+
+
+####################################################
+## 6. Modules
+####################################################
+
+# You can import modules
+import math
+print(math.sqrt(16)) # => 4
+
+# 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
+# defines a module.
+import math
+dir(math)
+
+
+####################################################
+## 7. Advanced
+####################################################
+
+# Generators help you make lazy code
+def double_numbers(iterable):
+ for i in iterable:
+ yield i + i
+
+# A generator creates values on the fly.
+# Instead of generating and returning all values at once it creates one in each
+# iteration. This means values bigger than 15 wont be processed in
+# double_numbers.
+# Note range is a generator too. Creating a list 1-900000000 would take lot of
+# time to be made
+# We use a trailing underscore in variable names when we want to use a name that
+# would normally collide with a python keyword
+range_ = range(1, 900000000)
+# will double all numbers until a result >=30 found
+for i in double_numbers(range_):
+ print(i)
+ if i >= 30:
+ break
+
+
+# Decorators
+# in this example beg wraps say
+# Beg will call say. If say_please is True then it will change the returned
+# message
+from functools import wraps
+
+
+def beg(target_function):
+ @wraps(target_function)
+ def wrapper(*args, **kwargs):
+ msg, say_please = target_function(*args, **kwargs)
+ if say_please:
+ return "{} {}".format(msg, "Please! I am poor :(")
+ return msg
+
+ return wrapper
+
+
+@beg
+def say(say_please=False):
+ msg = "Can you buy me a beer?"
+ 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 :(
+```
+
+## Ready For More?
+
+### 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/)
+* [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/)
+* [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/)
+
+### 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)