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diff --git a/python.html.markdown b/python.html.markdown index 2e7fd8be..0de51285 100644 --- a/python.html.markdown +++ b/python.html.markdown @@ -2,26 +2,30 @@ language: python contributors: - ["Louie Dinh", "http://ldinh.ca"] - - ["Amin Bandali", "http://aminbandali.com"] + - ["Amin Bandali", "https://aminb.org"] - ["Andre Polykanine", "https://github.com/Oire"] - ["evuez", "http://github.com/evuez"] + - ["asyne", "https://github.com/justblah"] + - ["habi", "http://github.com/habi"] 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] +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/). +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. +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. ```python @@ -33,7 +37,7 @@ allow you to write Python 3 code that will run on Python 2, so check out the Pyt """ #################################################### -## 1. Primitive Datatypes and Operators +# 1. Primitive Datatypes and Operators #################################################### # You have numbers @@ -50,41 +54,42 @@ allow you to write Python 3 code that will run on Python 2, so check out the Pyt 5 / 2 # => 2 # To fix division we need to learn about floats. -2.0 # This is a float +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 # => 1 +5.0 // 3.0 # => 1.0 # works on floats too -5 // 3 # => -2 --5.0 // 3.0 # => -2.0 +-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 + +11 / 4 # => 2.75 ...normal division +11 // 4 # => 2 ...floored division # Modulo operation -7 % 3 # => 1 +7 % 3 # => 1 # Exponentiation (x to the yth power) -2**4 # => 16 +2 ** 4 # => 16 # Enforce precedence with parentheses (1 + 3) * 2 # => 8 # Boolean Operators # Note "and" and "or" are case-sensitive -True and False #=> False -False or True #=> True +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 +0 and 2 # => 0 +-5 or 0 # => -5 +0 == False # => True +2 == True # => False +1 == True # => True # negate with not not True # => False @@ -123,12 +128,15 @@ not False # => True # A string can be treated like a list of characters "This is a string"[0] # => 'T' -#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. +# 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) +z = "The items in the basket are %s and %s" % (x, y) # A newer way to format strings is the format method. # This method is the preferred way @@ -164,20 +172,21 @@ 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! +print "I'm Python. Nice to meet you!" # => I'm Python. Nice to meet you! # 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 +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 +some_var = 5 # Convention is to use lower_case_with_underscores some_var # => 5 # Accessing a previously unassigned variable is an exception. @@ -194,14 +203,14 @@ 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 @@ -223,21 +232,21 @@ li[2:] # => [4, 3] # Omit the end li[:3] # => [1, 2, 4] # Select every second entry -li[::2] # =>[1, 4] +li[::2] # =>[1, 4] # Reverse a copy of the list -li[::-1] # => [3, 4, 2, 1] +li[::-1] # => [3, 4, 2, 1] # Use any combination of these to make advanced slices # li[start:end:step] # Remove arbitrary elements from a list with "del" -del li[2] # li is now [1, 2, 3] +del li[2] # li is now [1, 2, 3] # You can add lists -li + other_li # => [1, 2, 3, 4, 5, 6] +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] +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] @@ -251,31 +260,29 @@ li.index(2) # => 1 li.index(7) # Raises a ValueError as 7 is not in the list # Check for existence in a list with "in" -1 in li # => True +1 in li # => True # Examine the length with "len()" -len(li) # => 6 - +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 -tup + (4, 5, 6) # => (1, 2, 3, 4, 5, 6) -tup[:2] # => (1, 2) -2 in tup # => True +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 -d, e, f = 4, 5, 6 # you can leave out the parentheses +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 # Tuples are created by default if you leave out the parentheses -g = 4, 5, 6 # => (4, 5, 6) +g = 4, 5, 6 # => (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 - +e, d = d, e # d is now 5 and e is now 4 # Dictionaries store mappings empty_dict = {} @@ -283,30 +290,33 @@ empty_dict = {} filled_dict = {"one": 1, "two": 2, "three": 3} # Look up values with [] -filled_dict["one"] # => 1 +filled_dict["one"] # => 1 # Get all keys as a list with "keys()" -filled_dict.keys() # => ["three", "two", "one"] +filled_dict.keys() # => ["three", "two", "one"] # Note - Dictionary key ordering is not guaranteed. # Your results might not match this exactly. # Get all values as a list with "values()" -filled_dict.values() # => [3, 2, 1] +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_dicts.items() # => [("one", 1), ("two", 2), ("three", 3)] + # Check for existence of keys in a dictionary with "in" -"one" in filled_dict # => True -1 in filled_dict # => False +"one" in filled_dict # => True +1 in filled_dict # => False # Looking up a non-existing key is a KeyError -filled_dict["four"] # 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("four", 4) # => 4 +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) @@ -317,47 +327,46 @@ filled_dict["four"] = 4 # now, filled_dict["four"] => 4 filled_dict.setdefault("five", 5) # filled_dict["five"] is set to 5 filled_dict.setdefault("five", 6) # filled_dict["five"] is still 5 - # Sets store ... well sets (which are like lists but can contain no duplicates) 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]) +some_set = set([1, 2, 2, 3, 4]) # some_set is now set([1, 2, 3, 4]) # 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]) # Since Python 2.7, {} can be used to declare a set -filled_set = {1, 2, 2, 3, 4} # => {1, 2, 3, 4} +filled_set = {1, 2, 2, 3, 4} # => {1, 2, 3, 4} # Add more items to a set -filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5} +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} +filled_set & other_set # => {3, 4, 5} # Do set union with | -filled_set | other_set # => {1, 2, 3, 4, 5, 6} +filled_set | other_set # => {1, 2, 3, 4, 5, 6} # Do set difference with - -{1, 2, 3, 4} - {2, 3, 5} # => {1, 4} +{1, 2, 3, 4} - {2, 3, 5} # => {1, 4} # Do set symmetric difference with ^ {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 -10 in filled_set # => False +2 in filled_set # => True +10 in filled_set # => False #################################################### -## 3. Control Flow +# 3. Control Flow #################################################### # Let's just make a variable @@ -367,12 +376,11 @@ some_var = 5 # 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. +elif some_var < 10: # This elif clause is optional. print "some_var is smaller than 10." -else: # This is optional too. +else: # This is optional too. print "some_var is indeed 10." - """ For loops iterate over lists prints: @@ -428,12 +436,12 @@ 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 + 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 @@ -441,20 +449,22 @@ with open("myfile.txt") as f: for line in f: print line + #################################################### -## 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) - return x + y # Return values with a return statement + 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 +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. +add(y=6, x=5) # Keyword arguments can arrive in any order. # You can define functions that take a variable number of @@ -462,7 +472,8 @@ add(y=6, x=5) # Keyword arguments can arrive in any order. def varargs(*args): return args -varargs(1, 2, 3) # => (1, 2, 3) + +varargs(1, 2, 3) # => (1, 2, 3) # You can define functions that take a variable number of @@ -470,14 +481,17 @@ varargs(1, 2, 3) # => (1, 2, 3) def keyword_args(**kwargs): return kwargs + # Let's call it to see what happens -keyword_args(big="foot", loch="ness") # => {"big": "foot", "loch": "ness"} +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) @@ -488,9 +502,10 @@ all_the_args(1, 2, a=3, b=4) prints: # Use * to expand positional args and use ** to expand keyword args. 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) +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) + # you can pass args and kwargs along to other functions that take args/kwargs # by expanding them with * and ** respectively @@ -499,54 +514,64 @@ def pass_all_the_args(*args, **kwargs): print varargs(*args) print keyword_args(**kwargs) + # 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) + # 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 +add_10(3) # => 13 # There are also anonymous functions -(lambda x: x > 2)(3) # => True -(lambda x, y: x ** 2 + y ** 2)(2, 1) # => 5 +(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] +map(add_10, [1, 2, 3]) # => [11, 12, 13] +map(max, [1, 2, 3], [4, 2, 1]) # => [4, 2, 3] -filter(lambda x: x > 5, [3, 4, 5, 6, 7]) # => [6, 7] +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] -[x for x in [3, 4, 5, 6, 7] if x > 5] # => [6, 7] +[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} #################################################### -## 5. Classes +# 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" @@ -561,7 +586,6 @@ class Human(object): # Initialize property 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) @@ -597,45 +621,46 @@ class Human(object): # Instantiate a class i = Human(name="Ian") -print i.say("hi") # prints out "Ian: hi" +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" +i.get_species() # => "H. sapiens" # Change the shared attribute Human.species = "H. neanderthalensis" -i.get_species() # => "H. neanderthalensis" -j.get_species() # => "H. neanderthalensis" +i.get_species() # => "H. neanderthalensis" +j.get_species() # => "H. neanderthalensis" # Call the static method -Human.grunt() # => "*grunt*" +Human.grunt() # => "*grunt*" # Update the property i.age = 42 # Get the property -i.age # => 42 +i.age # => 42 # Delete the property del i.age i.age # => raises an AttributeError - #################################################### -## 6. Modules +# 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 +print floor(3.7) # => 3.0 # You can import all functions from a module. # Warning: this is not recommended @@ -643,9 +668,11 @@ from math import * # You can shorten module names import math as m -math.sqrt(16) == m.sqrt(16) # => True + +math.sqrt(16) == m.sqrt(16) # => True # you can also test that the functions are equivalent from math import sqrt + math.sqrt == m.sqrt == sqrt # => True # Python modules are just ordinary python files. You @@ -655,38 +682,94 @@ math.sqrt == m.sqrt == sqrt # => True # You can find out which functions and attributes # defines 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. + + #################################################### -## 7. Advanced +# 7. Advanced #################################################### -# Generators help you make lazy code +# Generators +# A generator "generates" values as they are requested instead of storing +# everything up front + +# 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 + + +# 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 + + +# We could instead use a generator to "generate" the doubled value as the item +# is being requested +def double_numbers_generator(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 xrange is a generator that does the same thing range does. -# Creating a list 1-900000000 would take lot of time and space to be made. -# xrange creates an xrange generator object instead of creating the entire list -# like range does. -# We use a trailing underscore in variable names when we want to use a name that -# would normally collide with a python keyword -xrange_ = xrange(1, 900000000) - -# will double all numbers until a result >=30 found -for i in double_numbers(xrange_): - print i - if i >= 30: + +# 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: 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]) +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]) +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 |