Python

Generic things:

The Python style guide is described in [PEP 8]

Modules

To add the location of your own modules to the python search path put it in the PYTHONPATH variable.

import sys

Provides a number of system variables

sys.argv

List of everything on the commandline. sys.argv[0] is the program itself.

import getopt

Module to parse the commandline arguments (sys.argv). Default available

import subprocess

Module to execute shell commands

In python2

import subprocess
exitcode = subprocess.call("<any command>")
commandoutput = subprocess.check_output("<any command>")

Use ("command",shell=True) to have the call work like it would on the commandline

In python3

import subprocess
CompletedProcess = subprocess.run("<any command>")

The CompletedProcess returned has (args, returncode, stdout, stderr)

Variables

Variables are always pointers to objects.

a = 2
b = 2

Point to the same object (the immutable integer '2')

Variables are local by default. If a routine has any assignment to a variable it is local. If you have defined a variable outside a routine and need assignments to it in the routine you have to declare it global explicitly. Variables are always pointers to objects.

a = 'a string'

def main():
    global a
    print(a)
    a = "This would fail with 'local variable 'a' referenced before assignment' if 'a' was not declared as global"

main()

[Geeks for Geeks] has as good page about this.