Python

Debugging, profiling and testing

See Python debugging, profiling and testing.

Piping functions

Everyone wants function piping to work in python, and so there are many implementations, e.g. here are two recent ones.

• Robert Yi, Flexible piping in python with Pipey
• Thuwarakesh Murallie, Use Pipe Operations in Python: Write Clean Codes Faster

No one has converged upon a standard. Also, how it is supposed to interact with generators? Many questions. Will ignore for now.

Pyodide

Python from the browser, including mathematical libraries.

• Documentation
• Pyodide blog

HT Emma Krantz for showing me this while solving my web interactive maths problems for me.

Idiom

Python people set great value on mastering python idiom. For example, python idiom for accessing a nullable member is a and a.x. I never remember this and always write getattr(a, 'x', None). Python idiom for ‘python person’ is pythonista. Python idiom for ‘idiomatic’ is pythonic. Much has been written on how to be pythonic.

I think learning can benefit also from negative examples. Here, may I proffer to you John Burkardt’s Hypershere integrals? That the least idiomatic python code I have seen. It is a line-for-line translation of C code into slow, verbose python. If you are going to do that, write C code.

ipython

The interactive python upgrade The python-specific part of jupyter, which can also run without jupyter. Long story. But think of it as a REPL, a CLI-style execution environment, that is a little friendlier and has colourisation and autocomplete and such. If sounds useful, read on

Pro tip: store config in .env

There are many, many python tools to load environment variables from local files. See .env..

String formatting/interpolation

## float precision
>>> print("{:.2f}".format(3.1415926))
3.14
## left padding
>>> print("{:0>2d}".format(5))
05

## power tip which the manual does not make clear:
## variable formatting of variable formatting
>>> pi = 3.1415926
>>> precision = 4
>>> print( "{:.{}f}".format( pi, precision ) )
3.1415

>>> name = "Fred"
>>> f"He said his name is {name!r}."
"He said his name is 'Fred'."

from datetime import datetime
datetime.utcnow().isoformat(timespec='seconds')

datetime.datetime.now().strftime("%Y%m%d-%H%M%S"))

from markdown import markdown
html_string = markdown("""
# Title

Paragraph. *emphasis*.
""")

Compiling and FFI

See Accelerating python.

Packaging and environments

See python packaging and environments.

Asynchrony in python

See asynchronous python.

functional-style programming

The style functional programming is often conceptually useful, and it can sometimes even be made performant in python.

toolz and the accelerated version cytoolz provide many useful functional abstractions which look fairly natural

ExitStack

If you are acquiring several context managers, there is a modern handy idion called ExitStack. For an explanation see On the Beauty of Python’s ExitStack by Nikolaus Rath.

with ExitStack() as cm:
    res1 = acquire_resource_one()
    cm.callback(release_resource, res1)
    # do stuff with res1
    res2 = acquire_resource_two()
    cm.callback(release_resource, res2)
    # do stuff with res1 and res2

Watching files for changes

Does this inotify solution work for non-Linux? Because macOS uses FSEvents and Windows uses I-don’t-even-know.

watchdog asserts that it is cross-platform (source).

emcrisostomo/fswatchclaims to be a universal filewatcher. There is a python frontend.

IDEs

I meant to list some, but in fact I only ever use VS Codium so that is the only current advice I would be able to give.

PyCharm also looks very good but I am hopelessly out of date on it

Typing

Python 3.6+ includes type hinting, and projects such as mypy support static analysis using type hints.¹ There were not many tutorials on the details of this at time of writing. Here’s one.

Short version: you go from this:

def fib(n):
  a, b = 0, 1
  while a < n:
    yield a
    a, b = b, a+b

to this:

def fib(n: int) -> Iterator[int]:
  a, b = 0, 1
  while a < n:
    yield a
    a, b = b, a+b

A version with actual corporate backing is pyright, which uses type checking to provide usability improvements in VS Codium. Might be useful.

Command line parsers

I am furious at how many options there are to build python CLI scripts. This is the new contender for most bike-shedded function now that the python web server world has calmed down a bit. AFAICT if you write some python software and don’t write a new command-line parsing library then you will be regarded as somehow feckless. If you are indeed so dissolute as to not make your own quirky artisinal command-line parser, you could try the listing at python CLIs.

Miscellaneous stuff I always need to look up

• Python exception hierarchy
• Python data structures

Progress meters

tqdm seems to be a de facto standard.

Basic version with a console progress bar:

pip install tqdm

Use it:

for x in tqdm(my_list):
    pbar.update(1)

Fancy integrated HTML display is available in jupyter. Install it:

pip install ipywidgets
jupyter nbextension enable --py widgetsnbextension
jupyter labextension install @jupyter-widgets/jupyterlab-manager

Use it:

%%capture
from tqdm import tqdm_notebook as tqdm
tqdm().pandas()

with tqdm(total=len(my_list)) as pbar:
    for x in my_list:
        pbar.update(1)

Misc recommendations

• Pext is a python extension to script interactive CLI processes things in a handy GUI.
• Let’s get weird: Hy is a lisp that compiles to python syntax trees

More Itertools A bunch of essential (IMO) extra iterators for python, essential for streaming work.

Python 2 vs 3

Are you very old? New to python 3? These days that makes you very old.

Sebastian Raschka, The key differences between Python 2.7.x and Python 3.x with examples

Incoming

• All About ‘Bank Python,’ a Finance-Specific Language Fork – The New Stack