Introduction to Python Programming Language

Ting-Yu Lin

tlin@mozilla.com

October 1, 2014

Overview

python_logo_official.png

Background

  • Creator: Guido van Rossum (now employed by Dropbox) GvR @ OSCON 2006, now age 58 Guido_van_Rossum_OSCON_2006.jpg
  • First appeared in 1991 (0.9.0)
  • Who uses Python? Mozilla, Google, Dropbox, MIT 6.00, etc. See List of Python software for more.
  • Mercurial. Mozilla Build system and automated testing environment.

Where does the Python name come from?

  • It's named after Monty Python's Flying Circus, a British television comedy by BBC from 1969 to 1974, not the snake.
    Flyingcircus_2.jpg
  • Python documents prefer to use spam, eggs, and ham instead of traditional foo, bar, and baz.

Python consists of:

  • Python core language
  • Python implementations
  • Python libraries

Python core languages (2 vs 3)

  • Python 2
  • Python 3
    • Backward-incompatible, released on December 2008
    • Python 3 is a polished Python, which is easier for newcomers. Latest 3.4.1.
    • Default string uses Unicode, print as a function, all classes are 'new' style class, etc.

Python language features

  • Typing system: dynamic, strong E.g. '3' + 5 generates TypeError exception
  • Multi-paradigm programming language
    • Object-oriented
    • Procedural (imperative)
    • Functional
    • Reflective
  • Python has a garbage collector. Everything is an object.

Python implementations

  • CPython: reference implementation. Written in C.
  • PyPy: implemented in RPython, translated to C. Interpreter has a Just-in-Time compiler.
  • Jython: implemented in Java. Run on JVM.
  • IronPython: implemented in C#. Run on .NET.

How to edit Python program?

  • Emacs
    emacs.png
  • Vim
    Vimlogo.png
  • Sublime Text 2
  • IDLE IDE (in standard Python distribution)
  • Other editors

Basic usage

The following examples are compatible with Python 2.7

Python shell

  • Linux and Mac should have Python installed.
  • Windows: Get Python from the official site, or from ActivePython.
  • Just type python in the shell 
    $ python
Python 2.7.8 (default, Aug 24 2014, 21:26:19)
[GCC 4.2.1 Compatible Apple LLVM 5.1 (clang-503.0.40)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
  • ipython: enhancement to python shell
    • pip install ipython

Run Python program

  • python hello_world.py
  • python -c "print 'Hello, Python!'"
  • python -m SimpleHTTPServer

Python as a calculator

>>> a = 2     # <type 'int'>
>>> b = 3.0   # <type 'float'>
>>> a / b
0.6666666666666666
>>> 2/3       # 0.6666666666666666 in Python 3
0

>>> a**100    # a to the power of 100
1267650600228229401496703205376L

>>> help('int')
Help on class int in module __builtin__:

class int(object)
 |  int(x[, base]) -> integer
...

String

  • String is immutable. 
    >>> c = "abcde" # same as single quote 'abcde'
>>> len(c)
5
>>> c[1]  # Python use 0-based indexing
'b'
>>> c[1:3]
'bc'

>>> print 'integer: {}, float: {}'.format(3, 5.12)
integer: 3, float: 5.120000

>>> '3' + 5
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects

List

  • List is mutable. 
    >>> d = [a, b, c]
>>> d
[2, 3.0, 'abcde']

>>> d[0]
2

>>> d[1:] + [['foo', 'bar'], 123]
[3.0, 'abcde', ['foo', 'bar'], 123]

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

>>> "1 2 3 4 5".split()
['1', '2', '3', '4', '5']

Variables

  • All variables in Python are references, not values. 
    >>> x = [0, 1, 2, 3]
>>> y = x
>>> y[0] = 'ha'
>>> x
['ha', 1, 2, 3]

Control structures

  • White spaces in Python are significant. 
    def f(n):
..for x in range(n):
....if x % 2 == 0:
......print x, 'is even.'
....else:
......print x, 'is odd.'
..else:                      # Run if the for loop is not break
......print 'ok'
..return ('a', 'b', 'c')     # Return a tuple

Tuple

  • Tuple is immutable.
    >>> x = f(4)
0 is even.
1 is odd.
2 is even.
3 is odd.
ok

>>> x
('a', 'b', 'c')

>>> a, b, c = x     # Tuple unpacking
>>> a
'a'
  • Tuple is like list. Why invent two similar types?

Dictionary

  • Mapping from key to value. Keys must be immutable.
    >>> e = {'Alice': 9372, 'Bob': 138, 'Carol': 7347}

>>> for (key, val) in e.items():
...   print key, ' ->', val
...

Bob -> 138
Alice -> 9372
Carol -> 7347

List comprehension

>>> x = []
>>> for i in range(10):
...   x.append(i**2)
>>> x
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> [i**2 for i in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

>>> map(lambda i: i**2, range(10))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

Boolean

  • True
    • not False
    • [1, 2, 3] and True
    • False or {"Bob": 138}
  • False
    • None
    • 0
    • Empty containers [], {}, (), etc.

Modules

  • If the file name is module_name.py, use import module_name
    >>> import random
>>> random.randint(1, 100) # Range from [1, 100]
43

>>> import itertools
>>> list(itertools.combinations('abcd', 3))
[('a', 'b', 'c'), ('a', 'b', 'd'), ('a', 'c', 'd'), ('b', 'c', 'd')]
  • How to distinguish executable and module?
    if __name__ == '__main__':
    # Do something when this file is executed as a program.

Example: Quick sort

def qsort(lst):
    """This implements quick sort."""
    if lst == []:
        return []

    pivot = lst[0]
    smaller = qsort([x for x in lst[1:] if x < pivot])
    bigger = qsort([x for x in lst[1:] if x >= pivot])
    return smaller + [pivot] + bigger

Decorator motivation

  • Print the function name when it is called. 
    def plus3(n):
    return n+3

def times2(n):
    return n*2

>>> plus3(4)
Calling plus3
7

>>> times2(3)
Calling times2
6

Decorator as a solution

  • Syntax sugar 
    @Log
def plus3(n):
    return n+3

def plus3(n):
    return n+3
plus3 = Log(plus3)

Decorator - Implement Log

def Log(func):
    def logged_func(*args, **kwargs):
        print "Calling", func.__name__
        return func(*args, **kwargs)
    return logged_func

class Log(object):
    def __init__(self, func):
        self.f = func

    def __call__(self, *args, **kwargs):
        print 'Calling', self.f.__name__
        return self.f(*args, **kwargs)

Python Libraries

  • batteries included: os, re, sqlite3, json, etc.
  • matplotlib, pycario
  • Most of the third party libraries support Python 3

What happened if typing import this in the Python interpreter?

  • import this
    >>> import this
The Zen of Python, by Tim Peters

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.

...
[skip 12 lines]
  • PEP 20 – The Zen of Python

How to debug Python programs?

  • pdb
    • python -m pdb some_prog.py
    • Insert import pdb; pdb.set_trace()
  • ipdb
    • pip install ipdb
    • Insert import ipdb; ipdb.set_trace()
  • Debugging Python Like a Boss

Dynamic features

duck.jpg

Duck typing

  • Duck typing allows polymorphism without inheritance!
    class Duck:
  def speak(self):
    print 'Quack!'

class Cat:
  def speak(self):
    print 'Meow!'

def happy(x):
  x.speak()

>>> d = Duck()
>>> c = Cat()
>>> happy(d)
Quack!
>>> happy(c)
Meow!
  • Usually inherit for implementation, not for interface.
  • Provided you can perform the job, we don't care who your parents are.

Reflection

  • The program can observe and modify its own structure and behavior at runtime.
    class C(object):
    def f(self):
        print 'f()'

>>> c = C()
>>> dir(c)
['__class__', '__delattr__', '__dict__', '__doc__', ..., 'f']

>>> c.val = 123
>>> dir(c)
['__class__', '__delattr__', '__dict__', '__doc__', ..., 'f', 'val']

>>> c.g()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'C' object has no attribute 'g'
>>> def g(): print 'g()'
>>> c.g = g
>>> c.g()
g()

Thank you. Any questions?

References

What tools are used to create this presentation?