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Adam Bard 2013-06-27 09:24:26 -07:00
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---
language: Python
author: Louie Dinh
author_url: http://ldinh.ca
---
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.
```Python
# Single line comments start with a hash.
""" Multiline comments can we written
using three "'s
"""
----------------------------------------------------
-- 1. Primitive Datatypes and Operators
----------------------------------------------------
# You have numbers
3 #=> 3
# Math is what you would expect
1 + 1 #=> 2
8 - 1 #=> 9
10 * 2 #=> 20
35 / 5 #=> 7
# Division is a bit tricky. It is integer division and floors the results automatically.
11 / 4 #=> 2
# Enforce precedence with parentheses
(1 + 3) * 2 #=> 8
# Boolean values are primitives
True
False
# negate with not
not True #=> False
not False #=> True
# Equality is ==
1 == 1 #=> True
2 == 1 #=> False
# Strings are created with " or '
"This is a string."
'This is also a string.'
# Strings can be added too!
"Hello " + "world!" #=> "Hello world!"
# A string can be treated like a list of characters
"This is a string"[0] #=> 'T'
# None is an object
None #=> None
----------------------------------------------------
-- 2. Variables and Collections
----------------------------------------------------
# Printing is pretty easy
print "I'm Python. Nice to meet you!"
# No need to declare variables before assigning to them.
some_var = 5 # Convention is to use lower_case_with_underscores for variables
some_var #=> 5
# Accessing a previously unassigned variable is an exception
some_other_var # Will raise 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]
# Access a list like you would any array
li[0] #=> 1
# Looking out of bounds is an IndexError
li[4] # Raises an IndexError
# Remove 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: li and other_li is left alone
# 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
# Dictionaries store mappings
empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {"one": 1, "two": 2, "three": 3}
# Look up values with []
filled_dict["one"] #=> 1
# Get all keys as a list
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
filled_dict.values() #=> [3, 2, 1] Note - Same as above regarding key ordering.
# Check for existence of keys in a dictionary with in
"one" in filled_dict #=> True
1 in filled_dict #=> False
# Sets store ... well sets
empty_set = set()
# Initialize a set with a bunch of values
filled_set = set([1,2,2,3,4]) # filled_set is now set([1, 2, 3, 4])
# Add more items to a set
filled_set.add(5) # filled_set is now set([1, 2, 3, 4, 5])
# Do set intersection with &
other_set = set([3, 4, 5 ,6])
filled_set & other_set #=> set([3, 4, 5])
# Do set union with |
filled_set | other_set #=> set([1, 2, 3, 4, 5, 6])
# Check for existence in a set with in
2 in filled_set #=> True
10 in filled_set #=> False
----------------------------------------------------
-- 3. Control Flow
----------------------------------------------------
# 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"]:
print "%s is a mammal" % animal # You can use % to interpolate formatted strings
"""
While loops go until a condition is no longer met.
prints:
0
1
2
3
"""
x = 0
while x < 4:
print x
x += 1 # Short hand for x = x + 1
# Handle exceptions with a try/except block
try:
raise IndexError("This is an index error") # Use raise to raise an error
except IndexError as e:
pass # Pass is just a no-op. Usually you would do recovery here.
----------------------------------------------------
-- 4. Functions
----------------------------------------------------
# Use def to create new functions
def add(x, y):
print "x is %s and y is %s" % (x, y)
return x + y # Return values with a return statement
# Calling functions with parameters
add(5, 6) #=> 11 and prints out "x is 5 and y is 6"
# Another way to call functions is with keyword arguments
add(y=6, x=5) # Equivalent to above. 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
def keyword_args(**kwargs):
return kwargs
# Let's call it to see what happens
keyword_args(big="foot", loch="ness") #=> {"big": "foot", "loch": "ness"}
# Python has first class functions
def create_adder(x):
def adder(y):
return x + y
return adder
# Let's create a new function that always adds 10 to the argument
add_10 = create_adder(10):
add_10(3) #=> 13
# There are also anonymous functions
(lambda x: x > 2)(3) #=> True
# 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
[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 can define classes with the class statement
class Human(): # By convention CamelCase is used for classes.
pass
# We subclass from object to get a "new-style class". All your code should do this.
class Human(object):
# A class attribute. It is shared by all instances of this class
species = "H. sapiens"
# Basic initializer
def __init__(self, name):
self.name = name # We are assigning the argument to the instance's name attribute
# A method. All methods take self as the first argument, including the initializer
def say(self, msg):
return "%s: %s" % (self.name, msg)
# A class method is shared among all instances
@classmethod
def get_species(cls):
return cls.species
# Static methods are called without a parameter reference to the class or instance
@staticmethod
def grunt():
return "*grunt*"
# Instantiate a class
h = Human(name="Harry")
print h.say("hi") # prints out "Harry: hi"
i = Human("Ian")
print i.say("hello") #prints out "Ian: hello"
# Call our class method
h.get_species() #=> "H. sapiens"
# Change the shared attribute
h.species = "H. neanderthalensis"
h.get_species() #=> "H. neanderthalensis"
i.get_species() #=> "H. neanderthalensis"
# Call the static method
Human.grunt() #=> "*grunt*"