Are you a frequent user of Python or IPython interactive shells? Do you find yourself constantly reimporting the same libraries (I’m looking at you from pprint import pprint 👀), or redefining the same helper functions?
This post will describe how to set up some scripts that will be automatically executed when you run python or ipython in your terminal.
Python
When the Python interpreter starts, it looks for a PYTHONSTARTUP environment variable. This can be set to point to a Python file which will be executed before you’re dropped into the Python interpreter.
So if you create a ~/.custom_startup.py with:
print("hello from custom_startup.py!")
Then execute export PYTHONSTARTUP=~/.custom_startup.py.
Running python in your terminal, you’ll now see hello from custom_startup.py printed before the >>> prompt in the interpreter:
$ python
Python 3.9.0 (default, Dec 1 2020, 10:32:01)
[Clang 12.0.0 (clang-1200.0.32.21)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
hello from custom_startup.py
>>>
IPython
The process for IPython is slightly different - there’s no environment variable to set, and IPython expects startup scripts (yes, that’s intentionally plural) to live in ~/.ipython/profile_default/startup/. There’s even a README in that folder with some brief instructions:
$ cat ~/.ipython/profile_default/startup/README
This is the IPython startup directory
.py and .ipy files in this directory will be run *prior* to any code or files
specified via the exec_lines or exec_files configurables whenever you load this profile.
Files will be run in lexicographical order, so you can control the execution order of files
with a prefix, e.g.::
00-first.py
50-middle.py
99-last.ipy
The keen-eyed amongst you will also have noticed the profile_default in the path above. This means you can have separate startup scripts for different IPython profiles. This is not a feature I use currently and therefore I won’t cover it in more detail in this blog, but if you’re interested then check out the IPython docs.
But why?
Now that you’ve created your (I)Python startup scripts, you can start adding some useful imports, custom functions or taking your Python interpreter experience to a whole new level.
Basic Imports
As mentioned in the intro, do you find yourself always using pprint to print out data structures in a nicer format? Or maybe you’re always using the os or math modules? Add these as imports to your startup script, and they’ll be immediately accessible in your Python interpreter.
import math
import os
from pprint import pprint
~/.custom_startup.py or ~/.ipython/profile_default/startup/00-custom.py
$ python
Python 3.9.0 (default, Dec 1 2020, 10:32:01)
[Clang 12.0.0 (clang-1200.0.32.21)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> math
<module 'math' from '/Users/craig.rosie/.pyenv/versions/3.9.0/lib/python3.9/lib-dynload/math.cpython-39-darwin.so'>
>>> os
<module 'os' from '/Users/craig.rosie/.pyenv/versions/3.9.0/lib/python3.9/os.py'>
>>> pprint
<function pprint at 0x1020afc10>
Look ma, no import errors!
Custom Functions
Maybe you find yourself writing the same helper functions over and over again. Or paging through your interpreter history trying to find the last time you wrote them. Wouldn’t it be great if they were just always available?
# previous imports excluded for brevity
def dir_no_dunder(module):
"""Like the dir() builtin, but only prints non-dunder attributes."""
pprint([attr for attr in dir(module) if not attr.startswith("_")])
~/.custom_startup.py or ~/.ipython/profile_default/startup/00-custom.py
$ python
Python 3.9.0 (default, Dec 1 2020, 10:32:01)
[Clang 12.0.0 (clang-1200.0.32.21)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> dir_no_dunder(math)
['acos',
'acosh',
'asin',
'asinh',
'atan',
'atan2',
'atanh',
...
Taking it to the Next Level
Now for something that will give your interpreter superpowers. The Rich library by Will McGugan is an excellent addition to your startup script:
...
from functools import partial
from rich import pretty, print, inspect
pretty.install()
inspectm = partial(inspect, methods=True)
~/.custom_startup.py or ~/.ipython/profile_default/startup/00-custom.py
For a start, you get nicely coloured and formatted output of data structures:
But perhaps the killer feature is inspect():
We can also create our own custom version of inspect(), which I’ve done using a partial, to give a function that shows you all the methods for a module or object: inspectm = partial(inspect, methods=True)
Enjoy your interpreter with your new-found superpowers! 🎉