Introduction to Python

(for C++ programmers)

Background Information

• History– created in December 1989 by Guido van Rossum

• Interpreted• Dynamically-typed language– no int, char, long nonsense

• Object Oriented• Current versions are 2.7.3 and 3.3.0– not backwards compatible

Terms

• Pythonic – hard to define– programming in a way that lends itself well to the

Python language• Pythonista – a person who codes in a pythonic

way

The Zen of Python

• Beautiful is better than ugly.• Explicit is better than implicit.• Simple is better than complex.• Complex is better than complicated.• Flat is better than nested.• Sparse is better than dense.• Readability counts.• Special cases aren't special enough to break the rules.• Although practicality beats purity.• Errors should never pass silently.• Unless explicitly silenced.• …

Syntax

• Indentation• Colon on the line before indented block• No Braces

Types of Quotes

• Double quote – “string”• Single quotes – ‘string’

• Triple quotes –

Let’s Dive In!

• Hello World• A more interesting Hello World

Data Types

• Numeric Types — int, float, long, complex• List – []• Tuple – () More About Tuples• Dictionary – {} dict• string• set, frozenset• Mutable vs. Immutable• Indexing (slicing, step slicing)• Heterogeneous

C++ <=> PythonC++ Python

NULL None

true True

false False

! not

&& and

|| or

while(true); while(True): pass

for

• iterate through stuff

for var in iterable:do_something

in

• Check for membership• Iterating through lists (as in the ‘for’ example)

is vs. ==

• Remember Python is an OOPL?• The ‘is’ keyword checks if two variable names

point to the same memory.• == checks the value. Uses the __eq__

function in classes.

The idiomatic way to perform an operation on all items in a list in C looks like this:for (i=0; i < mylist_length; i++)

{ do_something(mylist[i]); } The direct equivalent in Python would be this:i = 0 while i < mylist_length:

do_something(mylist[i]) i += 1 That, however, while it works, is not considered Pythonic. It's not an idiom the

Python language encourages. We could improve it. A typical idiom in Python to generate all numbers in a list would be to use something like the built-in range() function:

for i in range(mylist_length): do_something(mylist[i])

This is however not Pythonic either. Here is the Pythonic way, encouraged by the language itself:

for element in mylist: do_something(element)

Pythonic Programming

Functions

• Functions are objects too

• Can return multiple values using tuple unpacking• Can pass in arbitrary number of elements

Useful Builtin Functions

• len()• range() and xrange()• enumerate()• map()• zip()• any() and all()• dir()• http://docs.python.org/py3k/library/functions

.html

List Comprehension and Generators

• List comprehension makes a list (subscriptable, iterable)• Generators• not a tuple• iterable• calculation gets done on demand (when iterated)• can’t subscript

Python 2 vs. Python 3

• print• range and xrange• raw_input and input• division– 2 does integer division. Use // to do floating– 3 does floating point. Use // to do integer

• str and bytes• Unicode• __future__• Full changes at http://bit.ly/djYOVa

Modules (Importing)

Gotchas

• Other languages have "variables“• Mixing tabs and spaces or inconsistent

indentation• no i++ notation; use i += 1• http://zephyrfalcon.org/labs/python_pitfalls.h

tml

Exceptions and Classes

• for another day• just know they exist

Tips and Tricks

• Swap Values • Chained comparisons

a < b < c is the same as a < b and b < c• easy_install - setuptools 0.6c11– http://pypi.python.org/pypi

• pdb - The Python Debugger• profile

Learning Python

• http://python.org/– Great documentation for every version!

• help()• ipython

– python shell– tab-completion– ? and ??

• http://bit.ly/2noLNE• http://pythontutor.com/• http://www.doughellmann.com/PyMOTW/index.html