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Adam Bard
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---
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language: Python
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author: Louie Dinh
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author_url: http://ldinh.ca
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---
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Python was created by Guido Van Rossum in the early 90's. It is now one of the most popular languages in existence. I fell in love with Python for it's syntactic clarity. It's basically executable pseudocode.
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```Python
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# Single line comments start with a hash.
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""" Multiline comments can we written
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using three "'s
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"""
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----------------------------------------------------
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-- 1. Primitive Datatypes and Operators
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----------------------------------------------------
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# You have numbers
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3 #=> 3
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# Math is what you would expect
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1 + 1 #=> 2
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8 - 1 #=> 9
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10 * 2 #=> 20
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35 / 5 #=> 7
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# Division is a bit tricky. It is integer division and floors the results automatically.
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11 / 4 #=> 2
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# Enforce precedence with parentheses
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(1 + 3) * 2 #=> 8
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# Boolean values are primitives
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True
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False
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# negate with not
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not True #=> False
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not False #=> True
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# Equality is ==
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1 == 1 #=> True
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2 == 1 #=> False
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# Strings are created with " or '
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"This is a string."
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'This is also a string.'
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# Strings can be added too!
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"Hello " + "world!" #=> "Hello world!"
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# A string can be treated like a list of characters
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"This is a string"[0] #=> 'T'
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# None is an object
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None #=> None
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----------------------------------------------------
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-- 2. Variables and Collections
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----------------------------------------------------
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# Printing is pretty easy
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print "I'm Python. Nice to meet you!"
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# No need to declare variables before assigning to them.
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some_var = 5 # Convention is to use lower_case_with_underscores for variables
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some_var #=> 5
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# Accessing a previously unassigned variable is an exception
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some_other_var # Will raise a NameError
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# Lists store sequences
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li = []
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# You can start with a prefilled list
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other_li = [4, 5, 6]
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# Add stuff to the end of a list with append
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li.append(1) #li is now [1]
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li.append(2) #li is now [1, 2]
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li.append(4) #li is now [1, 2, 4]
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li.append(3) #li is now [1, 2, 4, 3]
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# Access a list like you would any array
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li[0] #=> 1
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# Looking out of bounds is an IndexError
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li[4] # Raises an IndexError
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# Remove elements from a list with del
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del li[2] # li is now [1, 2, 3]
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# You can add lists
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li + other_li #=> [1, 2, 3, 4, 5, 6] - Note: li and other_li is left alone
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# Concatenate lists with extend
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li.extend(other_li) # Now li is [1 ,2 ,3 ,4 ,5 ,6]
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# Check for existence in a list with in
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1 in li #=> True
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# Examine the length with len
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len(li) #=> 6
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# Tuples are like lists but are immutable
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tup = (1, 2, 3)
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tup[0] #=> 1
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tup[0] = 3 # Raises a TypeError
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# Dictionaries store mappings
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empty_dict = {}
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# Here is a prefilled dictionary
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filled_dict = {"one": 1, "two": 2, "three": 3}
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# Look up values with []
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filled_dict["one"] #=> 1
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# Get all keys as a list
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filled_dict.keys() #=> ["three", "two", "one"] Note - Dictionary key ordering is not guaranteed. Your results might not match this exactly.
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# Get all values as a list
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filled_dict.values() #=> [3, 2, 1] Note - Same as above regarding key ordering.
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# Check for existence of keys in a dictionary with in
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"one" in filled_dict #=> True
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1 in filled_dict #=> False
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# Sets store ... well sets
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empty_set = set()
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# Initialize a set with a bunch of values
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filled_set = set([1,2,2,3,4]) # filled_set is now set([1, 2, 3, 4])
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# Add more items to a set
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filled_set.add(5) # filled_set is now set([1, 2, 3, 4, 5])
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# Do set intersection with &
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other_set = set([3, 4, 5 ,6])
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filled_set & other_set #=> set([3, 4, 5])
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# Do set union with |
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filled_set | other_set #=> set([1, 2, 3, 4, 5, 6])
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# Check for existence in a set with in
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2 in filled_set #=> True
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10 in filled_set #=> False
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----------------------------------------------------
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-- 3. Control Flow
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----------------------------------------------------
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# Let's just make a variable
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some_var = 5
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# Here is an if statement. INDENTATION IS SIGNIFICANT IN PYTHON!
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# prints "some var is smaller than 10"
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if some_var > 10:
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print "some_var is totally bigger than 10."
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elif some_var < 10: # This elif clause is optional.
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print "some_var is smaller than 10."
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else: # This is optional too.
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print "some_var is indeed 10."
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"""
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For loops iterate over lists
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prints:
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dog is a mammal
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cat is a mammal
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mouse is a mammal
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"""
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for animal in ["dog", "cat", "mouse"]:
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print "%s is a mammal" % animal # You can use % to interpolate formatted strings
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"""
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While loops go until a condition is no longer met.
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prints:
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0
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1
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2
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3
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"""
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x = 0
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while x < 4:
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print x
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x += 1 # Short hand for x = x + 1
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# Handle exceptions with a try/except block
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try:
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raise IndexError("This is an index error") # Use raise to raise an error
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except IndexError as e:
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pass # Pass is just a no-op. Usually you would do recovery here.
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----------------------------------------------------
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-- 4. Functions
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----------------------------------------------------
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# Use def to create new functions
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def add(x, y):
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print "x is %s and y is %s" % (x, y)
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return x + y # Return values with a return statement
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# Calling functions with parameters
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add(5, 6) #=> 11 and prints out "x is 5 and y is 6"
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# Another way to call functions is with keyword arguments
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add(y=6, x=5) # Equivalent to above. Keyword arguments can arrive in any order.
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# You can define functions that take a variable number of positional arguments
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def varargs(*args):
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return args
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varargs(1, 2, 3) #=> (1,2,3)
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# You can define functions that take a variable number of keyword arguments
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def keyword_args(**kwargs):
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return kwargs
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# Let's call it to see what happens
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keyword_args(big="foot", loch="ness") #=> {"big": "foot", "loch": "ness"}
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# Python has first class functions
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def create_adder(x):
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def adder(y):
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return x + y
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return adder
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# Let's create a new function that always adds 10 to the argument
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add_10 = create_adder(10):
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add_10(3) #=> 13
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# There are also anonymous functions
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(lambda x: x > 2)(3) #=> True
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# There are built-in higher order functions
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map(add_10, [1,2,3]) #=> [11, 12, 13]
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filter(lambda x: x > 5, [3, 4, 5, 6, 7]) #=> [6, 7]
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# We can use list comprehensions for nice maps and filters
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[add_10(i) for i in [1, 2, 3]] #=> [11, 12, 13]
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[x for x in [3, 4, 5, 6, 7] if x > 5] #=> [6, 7]
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----------------------------------------------------
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-- 5. Classes
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----------------------------------------------------
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# We can define classes with the class statement
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class Human(): # By convention CamelCase is used for classes.
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pass
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# We subclass from object to get a "new-style class". All your code should do this.
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class Human(object):
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# A class attribute. It is shared by all instances of this class
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species = "H. sapiens"
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# Basic initializer
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def __init__(self, name):
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self.name = name # We are assigning the argument to the instance's name attribute
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# A method. All methods take self as the first argument, including the initializer
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def say(self, msg):
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return "%s: %s" % (self.name, msg)
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# A class method is shared among all instances
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@classmethod
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def get_species(cls):
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return cls.species
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# Static methods are called without a parameter reference to the class or instance
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@staticmethod
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def grunt():
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return "*grunt*"
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# Instantiate a class
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h = Human(name="Harry")
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print h.say("hi") # prints out "Harry: hi"
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i = Human("Ian")
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print i.say("hello") #prints out "Ian: hello"
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# Call our class method
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h.get_species() #=> "H. sapiens"
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# Change the shared attribute
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h.species = "H. neanderthalensis"
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h.get_species() #=> "H. neanderthalensis"
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i.get_species() #=> "H. neanderthalensis"
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# Call the static method
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Human.grunt() #=> "*grunt*"
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