Python Reference

Basic Operations

• Use variable = value to assign a value to a variable.
• Use print first, second, third to display values.
• Python counts from 0, not from 1.
• # starts a comment.
• Statements in a block must be indented (usually by four spaces).
• help(thing) displays help.
• len(thing) produces the length of a collection.
• [value1, value2, value3, ...] creates a list.
• list_name[i] selects the i'th value from a list.

Control Flow

• Create a for loop to process elements in a collection one at a time:

  for variable in collection:
      ...body...
  

• Create a conditional using if, elif, and else:

  if condition_1:
      ...body...
  elif condition_2:
      ...body...
  else:
      ...body...
  

• Use == to test for equality.
• X and Y is only true if both X and Y are true.
• X or Y is true if either X or Y, or both, are true.
• Use assert condition, message to check that something is true when the program is running.

Functions

• Define a new function with the name do_stuff that takes parameters arg_1, arg_2, and arg_3:

  def do_stuff(arg_1, arg_2, arg_3):
      ...function body indented 4 spaces...
      ...optionally return a result...
      return ...value...
  

  Note: Functions that do not return a result may omit the return statement at the end.

• Define a function that includes an optional parameter with a default value of 1:

  def do_stuff(arg_1, arg_2, arg_3, arg_opt=1):
      ...function body...
  

• Call a function using do_stuff(...values...).

Libraries

• Import a library into a program using import libraryname.
  • Also called a "module" in Python.
• The sys library contains:
  • sys.argv: the command-line arguments a program was run with.
  • sys.stdin, sys.stdout: standard input and output.
• glob.glob(pattern) returns a list of files whose names match a pattern.

Arrays

• import numpy to load the NumPy library.
• array.shape gives the shape of an array.
• array[x, y] selects a single element from an array.
• low:high specifies a slice including elements from low to high-1.
• array.mean(), array.max(), and array.min() calculate simple statistics.
• array.mean(axis=0) calculates statistics across the specified axis.