Python 101 - Session 1

ENV 859 - Advanced GIS John Fay, Fall 2017

Materials

• Whirlwind Tour of Python, Jake VanderPlas (2016)
  • Book: http://www.oreilly.com/programming/free/a-whirlwind-tour-of-python.csp
  • Free PDF: http://www.oreilly.com/programming/free/files/a-whirlwind-tour-of-python.pdf
  • Interactive: http://nbviewer.jupyter.org/github/jakevdp/WhirlwindTourOfPython/blob/master/00-Introduction.ipynb
  • GitHub: https://github.com/jakevdp/WhirlwindTourOfPython (main) and https://github.com/ENV859/WhirlwindTourOfPython (our copy)

Prep

• Ensure ArcGIS Desktop and ArcGIS Pro are installed on your VCM image
• See ArcGIS Desktop Post-Install and ArcGIS Pro post-install docs
• Fork WhirlwindTourOfPython and clone to your machine using GitHub desktop
  • Doc on how to fork repositories: link

Introduction

​ Source

• Dive in: Python command line in ArcMap

  • Hello world

• What is scripting? Link

• Python vs other languages

  • The Zen of Python; "Pythonic"

Set-up: Where does Python live and how to run Python code

​ Source and here

• ### Where does Python live?

  • Python 2.7 with ArcGIS Desktop C:\Python27\ArcGIS10.5\python.exe
  • Python 3.5 with ArcGIS Pro: C:\Program Files\ArcGIS\Pro\bin\Python\envs\arcgispro-py3\python.exe
  • Also: Anaconda Python* - https://anaconda.org/anaconda/python

• ### Running Python code: Hello World

  • Python Command shell
    • C:\Python27\ArcGIS10.5\python.exe
  • IPython shell
    • C:\Python27\ArcGIS10.5\scripts\ipython.exe
  • Python scripts...
  • Jupyter notebooks...
  • IDEs: PythonWin

• ### Quick Tour of Jupyter notebooks

  ​ Source and here

  • How it runs: Server, content (notebooks), browser, JavaScript
  • Open notebooks (.ipynb) via main page
  • Markdown vs code
  • Executing code
  • Create a new notebook
  • Add a cell
  • Write code
  • Execute code
  • Closing a notebook
  • Closing the Jupyter server

Basic Python Syntax

​ Source

• Reading Python code

• Comments

• Whitespace

• End of lines

Basic Python Semantics: Variables and Objects

​ Source

• ### Python variables

  • Variable assignment with =
  • Mutable vs immutable objects
  • Two variable names can point to the same mutable object
    • Change one and the other changes

• ### Objects and data types

  • In Python, the variable's value assigns its data type
  • type() function
  • help() function

Basic Python Semantics: Operators

​ Source

• Python as a calculator
  • Addition, subtraction, multiplication, division (true and floor)
  • Modulus
  • Exponentiation (powers)
• Bitwise operations (not covered...)
• Assignment operations
  • Assigning and re-assigning values with =
  • Augmented assignment values: +=, -=, *=, */,...
• Comparison operations
  • returns a Boolean
  • is equal to, is greater than...: '==, >,>-,!='
• Boolean operations
  • and, or, not
  • Boolean vs bitwise
• Identity and membership operators
  • is infers object identity, not equality; two variables can be equal, but not the same object
    • If x is y is true, then changing one will change the other
  • in checks for membership

Built-In Types: Simple Values/Scalars

​ Source

• Integers
  • "fixed" vs "variable" precision (short, long)
    • a = 2; type(a) -> 'int'
    • b = 2 ** 250; type(b) --> 'long'
  • Integer division, v2 vs v3...
• Floating-point
  • standard vs decimal notation
  • floating point precision gotcha
• Complex - not covered...
• Strings
  • Quotes
  • string functions (Jupyter's tab completion)
  • Concatenation
  • String indexing and slicing (to be continued...)
• None
• Booleans
  • Deriving vs creating
  • True and False keywords
  • 0 is false, all other numbers are true: bool(0) vs bool(-23)
  • "" is false, all other string is true: bool("") vs bool('false')