Lesson 1 - Introduction

XKCD #353

• Python is consistently ranked among the top programming languages

  • Currently second on GitHub (active projects) behind JavaScript

    • Overtook Java in 2017

  • Heavily used in data science and scientific computing

• Extensive ecosystem

  • PyPI contains over 151,000 packages (September 2018)

• Simple

  • Or as complicated as you need

• Integrates with C libraries

  • C API, ctypes, CFFI, Cython, etc

• In demand

  • 67% increase in Python jobs on Indeed.com between 2012 and 2016

• Created by Guido van Rossum

  • From the Netherlands
  • At Dropbox since December 2012
  • Formally at Google, NIST, CNRI
  • Resigned as BDFL (Benevolent Dictator For Life) in July 2018

• Started in December 1989

  • Guido was trying to stay busy while his office was closed for Christmas

  • Named after Monty Python’s Flying Circus

    • Python documentation has a lot of references to Monty Python.

      • “eggs” and “spam” is commonly used instead of “foo” and “bar”

      • Example from Input and Output in the official Python tutorial

        >>> print('We are the {} who say "{}!"'.format('knights', 'Ni'))
        We are the knights who say "Ni!"
        

• CPython

  • Most popular
  • Reference Implementation

• PyPy

  • Fast version of Python written in RPython

    • statically-typed subset of CPython

• Jython

  • Python that runs on a JVM

• IronPython

  • Python for .NET

• Cython

  • Compiles Python code to C
  • Not to be confused with CPython

• Several others

• The current version is 3.7 (3.8 scheduled for October 2019)

  • Python 3 is not backwards compatible with Python 2
  • Compatible code can be written, easier with the six module

• Python 2.7 was released in July 2010

  • Slated for retirement in 2020
  • Only receives bugfixes and backports to assist in transition to Python 3
  • A lot of code is still Python 2, so we’ll see it for a long time
  • Tauthon is a project to create an unofficial Python 2.8

• EL6: 2.6.6

  • Later versions available with Software Collections
  • Python 3.4 available in EPEL

• EL7: 2.7.5

  • Later versions available with Software Collections
  • Python 3.4 available in EPEL

• EL8: 3.x

  • RHEL 8 will be Python 3 only

• PEP 8 is the official style Guide

  • Covers naming, indentation, spacing, etc
  • Some areas are up for interpretation
  • The goal is consistency and readability, not strict adherence

• Summary

  • Indent with 4 spaces, no tabs
  • Constants in UPPERCASE
  • Class names in CapWords
  • Almost everything else in lowercase_with_underscores

• Console

  • vim

    • Set tab to 4 space in .vimrc

      set tabstop=4
      set shiftwidth=4
      set expandtab
      

  • Emacs

• GUI

  • gedit
  • Atom
  • Sublime Text

• VSCode with Python extension

  • Written in Typescript
  • By Microsoft (Open source)

• Spyder

  • Written in Python
  • Targeted at the scientific community

• PyCharm

  • Written in Java
  • Commercial and community versions

• IDLE

  • Written in Python
  • Basic IDE include with Python

• pylint

  • Dynamic linter (Loads code)
  • Looks for style and functional errors

• pyflakes

  • Static style checker

• pycodestyle

  • Checks code for PEP 8 compliance

• pydocstyle

  • Checks docstrings for PEP 257 compliance

• mccabe

  • Checks code complexity

• flake8

  • Wrapper for pyflakes, pycodestyle, and mccabe

• Debuggers

  • pdb

    • Built in debugger

  • Most IDEs have graphical debuggers

• Documentation

  • Sphinx

    • Tool for creating documentation from reStructuredText
    • Used (with Hieroglyph) to create these slides
    • Easy API documentation with autodoc

• Shells

  • REPL (Read–Eval–Print Loop)

    • Built-in console

  • IPython

    • Feature-rich console

• Python Documentation

  • https://docs.python.org

• PEP 8 Style Guide

  • https://www.python.org/dev/peps/pep-0008/

• Python Package Index (PyPI)

  • https://pypi.python.org

• Talk Python to Me Podcast

  • https://talkpython.fm

• Exercism - Coding exercises (beginner to intermediate)

  • http://exercism.io/languages/python

• CheckIO - Coding Game (intermediate to advanced)

  • https://py.checkio.org/

• See which version of Python you’re running

  $ python -V
  Python 2.7.12
  

• Python 2:

  #!/usr/bin/env python
  
  # This is a comment
  print "Hello, world"
  

• Python 3:

  #!/usr/bin/env python
  
  # This is a comment
  print("Hello, world")
  

• Execute script

  $ chmod +x first_script.py
  $ ./first_script.py
  Hello, world
  $ python first_script.py
  Hello, world
  

• Tells the shell where to find the Python interpreter

• #!/usr/bin/env python

  • Uses first python in $PATH

• #!/usr/bin/python

  • Uses the system default python

• #!/usr/bin/python3.6

  • Uses a specific python

• All options are valid, just different

• Enter Console

  $ python
  Python 2.7.12 (default, Aug  9 2016, 15:48:18)
  [GCC 6.1.1 20160621 (Red Hat 6.1.1-3)] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>>
  

• Exit Console

  • exit() or ctrl-d

  >>> exit()
  $
  

• Enter console after running script

  • Useful for troubleshooting

  $ python -i first_script.py
  

• Hello, world

  • Python 2:

    >>> print "Hello, world"
    Hello, world
    

  • Python 3:

    >>> print("Hello, world")
    Hello, world
    

• In Python 2 print is a statement

  • No parentheses

• In Python 3, print() is a function

  • Requires parentheses

• To use the print() in Python 2, hop in a Delorean

  >>> from __future__ import print_function
  >>> print("Hello, world")
  Hello, world
  

• Bonus Question:

  • Why does print("Hello, world") produce the same output in Python 2 and 3?
  • Hint: Try print(1, 2)

• In Python, white space matters

  • Standard indent is 4 space

  • No curly braces for code blocks

  • No semicolons at the end of statements

    • Still used sometimes for one-liners

  • Indent to start a code block

  • Dedent to end a code block

  • Indents must match!

• Notice the second set of dots? The interpreter is waiting for a dedent.

  >>> for n in [1, 2, 3]:
  ...     print n
  ...
  1
  2
  3
  

• Comments are preceded by a pound sign #

• Comments can occur on separate lines or at the end of a line

  # This is a comment
  print('something')  # This is an inline comment
  

• Docstrings appear at the beginning of a file or object definition

  • Generally in triple-double quotes

  #!/usr/bin/env python
  """
  This is where you would describe the script
  It is also a good place to include contact and copyright information
  """
  

• Appending a backslash \ to the end of a line will cause it to continue to the next line

• Strings are automatically concatenated

  >>> myString = "This is the string the never ends. " \
  ...     "It just goes on and on, my friend."
  >>> myString
  "This is the string the never ends. It just goes on and on, my friend."
  

• Open parentheses, braces and brackets imply a line continuation

  >>> print(
  ...       'something')
  something
  

• Additional functions, classes, and objects can be imported from modules
• Modules are simply a Python file

• Packages are collections of modules or subpackages

  • Example: urllib.request is a module in the urllib package

• A lot of very useful modules and packages are shipped with Python

  • This is called The Python Standard Library

• Many other modules and packages can be found online

  • The biggest repository is The Python Package Index
  • Many of the most popular packages are available from Red Hat or EPEL

• Imports are managed with the import statement

• Import a module

  >>> import sys
  >>> sys.version_info
  sys.version_info(major=3, minor=5, micro=1, releaselevel='final', serial=0)
  

• Import module with a different name

  >>> import sys as system
  >>> system.version_info
  sys.version_info(major=3, minor=5, micro=1, releaselevel='final', serial=0)
  

• Import objects from a module

  • Notice the module name is not given when calling version_info

  >>> from sys import version, version_info
  >>> version_info
  sys.version_info(major=3, minor=5, micro=1, releaselevel='final', serial=0)
  

• Sometime you will see a star in the import line

  >>> from sys import *
  

  • Don’t do this!

  • This means import “everything” from a module

    • What “everything” includes is configurable with a module

  • Wildcard imports can lead to unexpected behavior

• The module search path is installation dependent

• The current path can be displayed with sys.path

  >>> sys.path
  ['', '/usr/lib64/python35.zip', '/usr/lib64/python3.5',
  '/usr/lib64/python3.5/plat-linux', '/usr/lib64/python3.5/lib-dynload',
  '/usr/lib64/python3.5/site-packages', '/usr/lib/python3.5/site-packages']
  

• When running a script, the directory the script is in is the first in the search path

  • This is not the same as the current working directory

• The path can be prepended with the PYTHONPATH environment variable

  $ PYTHONPATH=/usr/share/superpython:/usr/share/superduperpython python
  

• A collection of packages and modules shipped with Python
• Provides standard solutions for common problems
• Provides standard interfaces for low level or OS-specific operations
• Refer to the documentation for a complete list

• Highlights

  • sys — System-specific parameters and functions
  • os — Miscellaneous operating system interfaces
  • glob — Unix style pathname pattern expansion
  • shutil — High-level file operations
  • re — Regular expression operations
  • random — Generate pseudo-random numbers
  • time — Time access and conversions (lower level)
  • datetime — Basic date and time types (higher level)

• The help() function, allows help access directly from the Python console

  • If the argument is a string, the topic will be searched for

  • Any other object will bring up help for that object type or class

    >>> help('tuple')
    >>> help(tuple)
    >>> help(myTuple)
    

• Help is also available from the command line with the pydoc command

  $ pydoc tuple
  

• Objects can also be inspected, using the dir() function

  • Implementation can vary by object type
  • Generally lists attributes and methods associated with an object