Python

Syntactic sugar for compiled code



Guido van Rossum and Python, by Sir Frederick Leighton

A Swiss army knife of coding tools. Good matrix library, general scientific tools, statistics tools, web server, art tools, but, most usefully, interoperation with everything else - It wraps C, C++, Fortran. It includes HTTP clients, parsers, API libraries, and all the other fruits of a thriving diverse community with a common trade language. Fast enough, easy to debug, garbage-collected. If some bit is too slow, you call into a compiled language, otherwise, you relax. A good default if you’d rather get stuff done than write code.

I typically do my stats and graphs in R. My user interface is javascript, My linear algebra library is fortran, and I use julia for my other scientific calculations. Python is the thread that stitches this Frankensteinisch monster together.

Of course, it could be better. clojure is more elegant, scala is easier to parallelise, julia prioritises science more highly, MATLAB is what your academic advisor is obsessed with…

But in terms of using a well-supported satisfactory and unexciting language that goes on your computer right now, and requires me to reinvent few wheels, and which is transferable across number crunching, web development, UIs, text processing, graphics and sundry other domains, and does not require heavy licensing costs, this one is a good starting point. See Hillel Wayne, The hard part of learning a language that lists all the main friction points that various languages have, and which I read as a low-key endorsement of the acceptable mediocrity of this particular language. What a pitch! Wow! Python for the win! Now, let’s look closer and see all the horrid things that are wrong with it.

Debugging, profiling and testing

See Python debugging, profiling and testing.

Idiom

Python people set great value on mastering python idiom. For example, python idiom for accessing a nullable member is a and a.x Python idiom for ‘python person’ is pythonista. Python idiom for ‘idiomatic’ is pythonic. Much has been written on how to be pythonic. But I think learning requires also negative examples. Here, may I proffer to you John Burkardt’s Hypershere integrals as an example of the least idiomatic python code I have seen?

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.

ipython config

Ipython config is per-default located in

(ipython locate)/profile_default/startup/

Autocomplete breaks

The ecosystem that support tab-completion is fragile and lackadaisical. Usually if autocomplete is broken it is because the sensitive dependencies of jedi are managed by cowboys.

Recent example iPython 7.19.0 autocomplete breaks with jedi <=0.17.0 and parso >=0.8.0.

The fix for that particular version was

conda install jedi==0.17.2

Pretty display of objects

Check out the rich display protocol which allows you to render objects as arbitrary graphics. This is especialy famous in the jupyter frontends. Some examples of how to use that are helpful. How to use this? The display api docs explain that you should implement methods on your objects such as, e.g., _repr_svg_.

This was what I used to create latex_fragment which can display arbitrary latex inline. This is how you use it:

def _figure_data(self, format):
    fig, ax = plt.subplots()
    ax.plot(self.data, 'o')
    ax.set_title(self._repr_latex_())
    data = print_figure(fig, format)
    # We MUST close the figure, otherwise
    # IPython’s display machinery
    # will pick it up and send it as output,
    # resulting in double display
    plt.close(fig)
    return data

# Here we define the special repr methods
# that provide the IPython display protocol
# Note that for the two figures, we cache
# the figure data once computed.

def _repr_png_(self):
    if self._png_data is None:
      self._png_data = self._figure_data('png')
    return self._png_data

For a non-graphical non-fancy terminal, you probably simply want nice formatting of big data structures.

from pprint import pprint, pformat
# display it
pprint(obj)
# get a formatted representation
pretty = pformat(obj)

You want to write your own pretty-printer, with correct indentation? Use tiles.

Pro tip: dotenv

dotenv allows easy configuration through OS environment variables or text files in the parent directory. You should probably use this. PRO-TIP: there are lots of packages with similar names. Make sure you install using this one

pip install python-dotenv
%% or
conda install -c conda-forge python-dotenv

Then you can be indifferent to whether files came from an FS config or an environment variable.

from dotenv import load_dotenv
load_dotenv()  # take environment variables from .env.
# Code of your application, which uses environment variables (e.g. from `os.environ` or
# `os.getenv`) as if they came from the actual environment.

String formatting/interpolation

Non obvious hacks

What a nightmare is that manual for the string formatting. While all the information I need is in there, it is arranged in perverse inversion of a comprehensible order as measured by the frequency and the priority with which I use it. See instead Marcus Kazmierczak’s cookbook.

Highlights:

## 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

f-strings

f-strings make things somewhat easier for python 3.6+ because they don’t need to mess around with naming things for the .format(foo=foo) call:

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

Timestamps

Why is a now timestamp in UTC not the first line in every academic research workbook/paper/data analysis? Because it’s tedious to look up the different bits.

Here you go:

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

Rendering HTML output

You have a quick and dirty chunk o’ HTML you need to output? You aren’t writing some damnable webapp with a nested hierarchy of template rendering and CSS integration into some design framework? You just want to pump out some markup?

I recommend yattag which is fast, simple, good and has a 1-page manual. It works real good.

Rendering markdown as HTML

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 expressive world of functional programming is often conceptually useful, and it can sometimes even be made performance in python.

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

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).

Python 2 vs 3

Are you old? New to python 3?

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

Neat python 3 stuff

Alex Rogozhnikov, Python 3 with pleasure highlights some tricks which landed in the python 3 lineage. There are a lot of those.

Useful for me is a friendlier python struct-like thing, the data class Geir Arne Hjelle explains.

Also nice, we can override module accessors.

Asynchrony is less awful (although still kind of awful).

Python 2 and 3 compatibility

TL;DR: I am no employee of giant enterprise-type business with a gigantic legacy code base, and so I don’t use python 2. My code is not python 2 compatible. Python 3 is more productive, and no-one is paying me to be less productive right now. Python 2 code is usually easy to port to python 3. It is possible to write code which is compatible with python 2 and 3, but then I would miss out on some of the work that has gone into making python 3 easier and better, and waste time porting elegant easy python 3 things to hard boring python 2 things.

🏗 six versus future.

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.

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. 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

Progress meters

tqdm seems to be a de facto standard.

Also works 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()

# Only this last bit is needed outside jupyter
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

  1. If I am going to that much trouble, I would possibly prefer to be using julia, which takes type-hinting further, by actually using it to JIT-compile optimised code.↩︎


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