pystupid

Some notes on Python syntax and more. These are on topics that are or were confusing to me, an R (and sometimes Fortran) user, while learning Python.

My intention is to document topics in this README.md file. But presently the issues page has more information.

Indexing objects

The 0 index for the first element is strange enough, but at least that matches C and probably more (some reasons for that here: https://developerinsider.co/why-does-the-indexing-of-array-start-with-zero-in-c/). But this "slice" behavior with numpy arrays and apparently other structures seems even stranger.

>>> import numpy as np
>>> x = [1, 2, 3]
>>> x[0]
1
>>> x[2]
3

OK, so to extract all 3 elements, I would expect:

>>> x[0:2]
[1, 2]

Nope! We need:

>>> x[0:3]
[1, 2, 3]

More on use of colon in Python here: https://www.askpython.com/python/examples/colon-in-python.

Debugging

The breakpoint() function is available in v3.7 and up. It seems to work well if the script is evaluated in the shell e.g.,

python3 somescript.py

Even if somescript.py loads the function that is to be debugged from another script (i.e., breakpoint() is in a separate function definition script) this works. If the work is done directly in a Python console, breakpoint() seems to be completely ignored. This differs from browser() behavior in R.

To step through a script without using breakpoint(), use this in the shell:

python3 -m pdb somescript.py 

(It seems breakpoint() uses the pdb package anyway.) But this will only step through the lines in somescript.py--don't try to use it to debug a function that is defined in a separate script and loaded in somescript.py.

Loading function definitions ("modules")

I'm still unsure about this task. Let's say I have a function somefunc() defined in a script somefuncscript.py. It can be imported with

from somefuncscript import somefunc

Note the omission of a file extension.

Strange behavior includes:

1. loading other functions defined in the script
2. challenges in loading functions saved in a different directory

On point 2, why can't we just include a file path, like below?

from ../funcs/somefuncscript import somefunc

There is probably a good reason, but I don't know why. There are a couple approaches discussed online that could be used, but I think copying the script into the working directory is simplest. There is a function in the shutil package (for shell utilities?) that does this.

shutil.copy('../funcs/somefuncscript.py', '.')
from somefuncscript import somefunc

Comparisons

In Python the behavior that R gives for && or || (evaluating following expressions depending on current one) is available but through and and or. So element-by-element comes with & and |, and the preferred programming approach comes with and and or. See https://docs.python.org/3/library/stdtypes.html#boolean-operations-and-or-not.

Note that there is an is operator needed, for example, for checking the class of an object.

type(x) is 'str'