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---
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language: python
author: Louie Dinh
author_url: http://ldinh.ca
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filename: learnpython.py
---
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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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Feedback would be highly appreciated! You can reach me at [@louiedinh](http://twitter.com/louiedinh) or louiedinh [at] [google's email service]
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Note: This article applies to Python 2.7 specifically, but should be applicable
to Python 2.x. Look for another tour of Python 3 soon!
```python
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# Single line comments start with a hash.
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""" Multiline strings can be written
using three "'s, and are often used
as comments
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"""
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####################################################
## 1. Primitive Datatypes and Operators
####################################################
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# You have numbers
3 #=> 3
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# Math is what you would expect
1 + 1 #=> 2
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8 - 1 #=> 7
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10 * 2 #=> 20
35 / 5 #=> 7
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# Division is a bit tricky. It is integer division and floors the results
# automatically.
5 / 2 #=> 2
# To fix division we need to learn about floats.
2.0 # This is a float
11.0 / 4.0 #=> 2.75 ahhh...much better
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# Enforce precedence with parentheses
(1 + 3) * 2 #=> 8
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# Boolean values are primitives
True
False
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# negate with not
not True #=> False
not False #=> True
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# Equality is ==
1 == 1 #=> True
2 == 1 #=> False
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# 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
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# Strings are created with " or '
"This is a string."
'This is also a string.'
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# Strings can be added too!
"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'
# % can be used to format strings, like this:
"%s can be %s" % ("strings", "interpolated")
# A newer way to format strings is the format method.
# This method is the preferred way
"{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")
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# None is an object
None #=> None
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####################################################
## 2. Variables and Collections
####################################################
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# Printing is pretty easy
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
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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
# if can be used as an expression
"yahoo!" if 1 > 2 else 2 #=> "yahoo!"
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# Lists store sequences
li = []
# You can start with a prefilled list
other_li = [4, 5, 6]
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# 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.
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# Access a list like you would any array
li[0] #=> 1
# Look at the last element
li[-1] #=> 3
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# Looking out of bounds is an IndexError
li[4] # Raises an IndexError
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# 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]
# Remove arbitrary 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
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
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
1 in li #=> True
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# Examine the length with len
len(li) #=> 6
# Tuples are like lists but are immutable.
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tup = (1, 2, 3)
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
# You can unpack tuples 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
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# Now look how easy it is to swap two values
e, d = d, e # d is now 5 and e is now 4
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# Dictionaries store mappings
empty_dict = {}
# Here is a prefilled dictionary
filled_dict = {"one": 1, "two": 2, "three": 3}
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# Look up values with []
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
"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 method is a safe way to add new key-value pair into dictionary
filled_dict.setdefault("five", 5) #filled_dict["five"] is set to 5
filled_dict.setdefault("five", 6) #filled_dict["five"] is still 5
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# Sets store ... well sets
empty_set = set()
# Initialize a set with a bunch of values
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some_set = set([1,2,2,3,4]) # filled_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}
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# Add more items to a set
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filled_set.add(5) # filled_set is now {1, 2, 3, 4, 5}
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# Do set intersection with &
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other_set = {3, 4, 5, 6}
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filled_set & other_set #=> {3, 4, 5}
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# Do set union with |
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filled_set | other_set #=> {1, 2, 3, 4, 5, 6}
# Do set difference with -
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{1,2,3,4} - {2,3,5} #=> {1, 4}
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# Check for existence in a set with in
2 in filled_set #=> True
10 in filled_set #=> False
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####################################################
## 3. Control Flow
####################################################
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# Let's just make a variable
some_var = 5
# 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:
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."
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"""
For loops iterate over lists
prints:
dog is a mammal
cat is a mammal
mouse is a mammal
"""
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for animal in ["dog", "cat", "mouse"]:
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# You can use % to interpolate formatted strings
print "%s is a mammal" % animal
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"""
While loops go until a condition is no longer met.
prints:
0
1
2
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3
"""
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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 # Shorthand for x = x + 1
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# Handle exceptions with a try/except block
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# Works on Python 2.6 and up:
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try:
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# Use raise to raise an error
raise IndexError("This is an index error")
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except IndexError as e:
pass # Pass is just a no-op. Usually you would do recovery here.
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# Works for Python 2.7 and down:
try:
raise IndexError("This is an index error")
except IndexError, e: # No "as", comma instead
pass
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####################################################
## 4. Functions
####################################################
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# 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
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# 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
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add(y=6, x=5) # 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):
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, as well
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def keyword_args(**kwargs):
return kwargs
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# Let's call it to see what happens
keyword_args(big="foot", loch="ness") #=> {"big": "foot", "loch": "ness"}
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# You can do both at once, if you like
def all_the_args(*args, **kwargs):
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print args
print kwargs
"""
all_the_args(1, 2, a=3, b=4) prints:
(1, 2)
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{"a": 3, "b": 4}
"""
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# When calling functions, you can do the opposite of varargs/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)
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# Python has first class functions
def create_adder(x):
def adder(y):
return x + y
return adder
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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
(lambda x: x > 2)(3) #=> True
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# 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]
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# 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]
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####################################################
## 5. Classes
####################################################
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# We subclass from object to get a class.
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class Human(object):
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# A class attribute. It is shared by all instances of this class
species = "H. sapiens"
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# Basic initializer
def __init__(self, name):
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# Assign the argument to the instance's name attribute
self.name = name
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# An instance method. All methods take self as the first argument
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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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# They are called with the calling class as the first argument
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@classmethod
def get_species(cls):
return cls.species
# A static method is called without a class or instance reference
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@staticmethod
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def grunt():
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return "*grunt*"
# Instantiate a class
i = Human(name="Ian")
print i.say("hi") # prints out "Ian: hi"
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j = Human("Joel")
print j.say("hello") #prints out "Joel: hello"
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# Call our class method
i.get_species() #=> "H. sapiens"
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# Change the shared attribute
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Human.species = "H. neanderthalensis"
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i.get_species() #=> "H. neanderthalensis"
j.get_species() #=> "H. neanderthalensis"
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# Call the static method
Human.grunt() #=> "*grunt*"
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```
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## Further Reading
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Still up for more? Try:
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[Learn Python The Hard Way](http://learnpythonthehardway.org/book/)
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[Dive Into Python](http://www.diveintopython.net/)
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[The Official Docs](http://docs.python.org/2.6/)