David Abel

0. Instructions

1. Basics

2. Functions

3. Dictionaries

4. OOP

5. Modules

6. Debugging

► 7. Tips & Tricks

Python Tutorial

This tutorial is intended as an introduction to Python for folks with some prior programming experience.

Section 7: Tips and Tricks: (no file)

A favorite trick of many python programmers is the list comprehension, which allows easy creation of lists in a single line (sometimes at the loss of readability). The syntax is:

>>> my_list = [<var> for <var> in <list>]

For instance:


        >>> my_list = [x for x in range(10)]

You're probably wondering why you would do this if you can simply call: list_of_ints = range(10). The neatest part about list comprehensions is adding conditionals, and nesting comprehensions. For instance:


        >>> my_list = [x for x in range(10) if x % 4 == 0] 

        >>> print my_list 

        [0,4,8]

Pretty cool! You can nest list comprehensions, too:


        >>> my_list = [[x for x in range(10) if x % 4 == 0] for j in range(3)] 

        >>> print my_list 

        [[0,4,8],[0,4,8],[0,4,8]]

Alternatively, you may multiplty a list by an integer N to create a new list with the old list repeated N times:


        >>> my_list = [0,4,8]*2 

        >>> print my_list 

        [0,4,8,0,4,8]

Another nice trick is the built in enumerate() function. This allows you to easily enumerate long lists of data. For instance:


        >>> states = ["Washington", "Oregon", "California"] 

        >>> my_list = list(enumerate(states)) 

        >>> print my_list 

        [(0, "Washington"), (1, "Oregon"), (2, "California")]

You can also specify a starting integer:


    >>> states = ["Washington", "Oregon", "California"] 

    >>> my_list = list(enumerate(states), start=3) 

    >>> print my_list 

    [(3, "Washington"), (4, "Oregon"), (5, "California")]

Another function you will likely find helpful is zip(). This takes two lists of equal length and returns a list of tuples consisting of the i-th element from each list paired together. For instance:


        >>> a = [0,1] 

        >>> b = [2,3] 

        >>> zipped = zip(a, b) 

        >>> print zipped
        [(0,2), (1,3)]

Python also allows for multiple assignments on one line:


        >>> x = 5 

        >>> y = 7 

        >>> x, y = y,x  

        >>> print x 

        7 

        >>> print y 

        5

This is also the best way to return multiple values from a function.


def get_key_value_pair(data): 

    # function body 

    return key, val 

K,V = get_key_value_pair(data)

And that's all I've got for now! Happy Pythoning! If you have any suggestions to improve this tutorial, email me at dmabel@google.com

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