Python

Python is...

• an interpreted, dynamically-typed, strongly-typed, multi-paradigm application language
• named for Monty Python's Flying Circus
• overseen by its creator, Guido van Rossum, a.k.a. the Benevolent Dictator For Life (BDFL)
  
• basically awesome

Python looks like...

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

# Copyright (c) 2011-2013 Christopher D. Lasher
#
# This software is released under the MIT License. Please see
# LICENSE.txt for details.


"""A collection of common utilities and convenient functions."""

import csv
import os.path


class SimpleTsvDialect(csv.excel_tab):
    """A simple tab-separated values dialect.


    This Dialect is similar to :class:`csv.excel_tab`, but uses
    ``'\\n'`` as the line terminator and does no special quoting.

    """
    lineterminator = '\n'
    quoting = csv.QUOTE_NONE


csv.register_dialect('simple_tsv', SimpleTsvDialect)


def make_csv_reader(csvfile, header=True, dialect=None, *args,
                    **kwargs):
    """Creates a CSV reader given a CSV file.

    :param csvfile: a file handle to a CSV file
    :param header: whether or not the file has header
    :param dialect: a :class:`csv.Dialect` instance
    :param *args: passed on to the reader
    :param **kwargs: passed on to the reader

    """
    if dialect is None:
        try:
            dialect = csv.Sniffer().sniff(csvfile.read(1024))
        except csv.Error:
            dialect = csv.excel
        csvfile.seek(0)
    if header:
        csv_reader = csv.DictReader(csvfile, dialect=dialect, *args,
                **kwargs)
    else:
        csv_reader = csv.reader(csvfile, dialect=dialect, *args,
                **kwargs)
    return csv_reader
            

Python is good for...

• Rapid development
• Modularity
• Readable code
• Documentation
• Glue (interfaces with C/C++ code)
• Web programming
• Science!!!

Python is bad for...

• Closed source
  • Counterpoint: awesome open source community
• Speed
  • Counterpoint: wrap C library or use NumPy/SciPy
• Preventing attribute access (no private/protected)
  • Counterpoint: “We're all adults here.”
  • Counterpoint: Code conventions (prefix with underscore, e.g., _private_var)

Python is an interpreted language

• Source code files processed at runtime by interpreter
• Interactive Intepreter (REPL)

Python Interpreter

• Source code organized as individual scripts, modules, packages, or entire applications
• Python interpreter invoked on a script or entry module
• Multiple interpreter implementations
  • CPython interpreter implementation usually what people mean by “Python”
  • Jython (implemented on JVM)
  • IronPython (implemented on .Net)
  • PyPy (Python interpreter implemented in Python)

Interactive Interpreters (REPL)

• Fantastic way for prototyping, exploring library APIs
• Usually can be invoked by simply entering python into terminal
• Alternative intepreters: IPython
• Web-based interpreters:
  • Skulpt
  • PythonAnywhere
  • Try Python! (on appspot)

Presentation code conventions

Python script/module

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

a = 1
b = 2
c = a + b
print(c)
            

Interactive Interpreter

>>> a = 1
>>> b = 2
>>> a + b
3
            

Mega-bonus "executables"

Brought to you by Online Python Tutor

Everything is an object

• Every entity is a first-class entity
• Objects have a specific “type”
• Variables are just references to objects

Variables are just named references

Reassigning a variable affects only that one

Immutable types

Basic Immutable Types

• None (Python's null)
• Numeric
  • Integer: -1, 9001, etc.
  • Float: 3.1459
  • Complex: 44+2j
• Boolean (technically subclass of integer): True or False
• Strings
• Tuples

Immutable types are immutable!

Python strings come in two flavors

• Unicode strings
• Byte strings

Unicode Strings

• Default in Python 3
• Handle crazy characters just fine

  
  msg = 'Wi nøt trei a høliday in Sweden this yër?'
                  

• In Python 2, prepend a u,

  
  msg = u"See the løveli lakes"
                  

Byte Strings

• default in Python 2
• only handle 256-bit encodings (think ASCII, ISO-8859-1); explicitly encode high-level bytes
• in Python 3 used for working directly with bytes
• in Python 3, create by prepending a b

  
  msg = b'See the l\xc3\xb8veli lakes'
                  

Quote Types

• Either single quote ' or double quote " may be used; functionally equivalent
• Triple-quoted strings (''' or """) preserve whitespace:

  
    >>> s = '''A triple-quoted
    ... string can
    ...     preserve
    ...         whitespace
    ... '''
    >>> s
    'A triple-quoted\nstring can\n    preserve\n        whitespace\n'
                    

Tuples are ordered collections

• Tuples are created using parentheses:

  
  ('a', 'b', 42)
                  

• Tuples can contain any other object:

  
  ((1, 2),)   # single-element tuple containing another
                  

• Objects do not have to be of the same type

Indexing

>>> a = (1, 2, (3, 4))
>>> a[0]   # access the first element
1
>>> a[-1]  # access the last element
(3, 4)
>>> a[-1][0]  # access the first element of the last element
3
            

Slicing

• Access multiple elements simultaneously by slicing
• Slicing syntax
  variable[start_index:stop_index:step_size]
• Any of start_index, stop_index, and step_size may be omitted
• Interval is zero-based, half-open; read a slice as "going from start_index up to, but not including stop_index

>>> a = (1, 2, 3)
>>> a[1:3]
(2, 3)
>>> a[1:]
(2, 3)
>>> a[:2]
(1, 2)
>>> a[::2]
(1, 3)
            

Lists

• Mutable collections of other objects
• vs. tuples: When in doubt, use lists
• Indexing and slicing work as tuples

>>> a = [81, 82, 83]
>>> a[0]
81
>>> a[1]
82
>>> a[:2]
[81, 82]
                

• Can reassign to lists

  
  >>> a
  [81, 82, 83]
  >>> a[0] = 'spam'
  >>> a
  ['spam', 82, 83]
                  

Mutable, mutable lists

>>> a
['spam', 82, 83]
>>> a.index('spam')
0
>>> del a[0]
>>> a
[82, 83]
>>> a.remove(83)
>>> a
[82]
>>> elem = a.pop()
>>> elem
82
>>> a
[]
>>> a.append(elem)
>>> a
[82]
>>> a.insert(0, 81)
>>> a
[81, 82]
>>> a.extend([83, 84, 85])
>>> a
[81, 82, 83, 84, 85]
>>> a[1:1] = ['happy', 'joy']
>>> a
[81, 'happy', 'joy', 82, 83, 84, 85]