Someone recently asked how to permanently change the prompt in the Python interactive REPL. The answer is you can point the PYTHONSTARTUP environment variable at a Python file, and that file will be executed every time you enter the interactive prompt.

I use this to import modules I often want to use, define helpers, and configure my command history.

In my .bashrc I have:

export PYTHONSTARTUP=~/.startup.py

Then my .startup.py file is:

# Ned's startup.py file, loaded into interactive python prompts.
# Has to work on both 2.x and 3.x

print("(.startup.py)")

import collections, datetime, itertools, math, os, pprint, re, sys, time
print("(imported collections, datetime, itertools, math, os, pprint, re, sys, time)")

pp = pprint.pprint

# A function for pasting code into the repl.
def paste():
    import textwrap
    exec(textwrap.dedent(sys.stdin.read()), globals())

# Readline and history support
def hook_up_history():
    try:
        # Not sure why this module is missing in some places, but deal with it.
        import readline
    except ImportError:
        print("No readline, use ^H")
    else:
        import atexit
        import os
        import rlcompleter

        history_path = os.path.expanduser(
            "~/.pyhistory{0}".format(sys.version_info[0])
        )

        def save_history(history_path=history_path):
            import readline
            readline.write_history_file(history_path)

        if os.path.exists(history_path):
            readline.read_history_file(history_path)

        atexit.register(save_history)

# Don't do history stuff if we are IPython, it has its own thing.
is_ipython = 'In' in globals()
if not is_ipython:
    hook_up_history()

# Get rid of globals we don't want.
del is_ipython, hook_up_history

A few things could us an explanation. The paste() function lets me paste code into the REPL that has blank lines in it, or is indented. Basically, I can copy code from somewhere, and use paste() to paste it into the prompt without having to fix those things first. Run paste(), then paste the code, then type an EOF indicator (Ctrl-D or Ctrl-Z, depending on your OS). The pasted code will be run as if it had been entered correctly.

The history stuff gives me history that persists across Python invocations, and keeps the Python 2 history separate from the Python 3 history. “pp” is very handy to have as a short alias.

Of course, you can put anything you want in your own .startup.py file. It’s only run for interactive sessions, not when you are running programs, so you don’t have to worry that you will corrupt important programs.