Python notes - semi-sorted

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Syntaxish: syntax and language · changes and py2/3 · decorators · importing, modules, packages · iterable stuff · concurrency · exceptions, warnings

IO: networking and web · filesystem

Data: Numpy, scipy · pandas, dask · struct, buffer, array, bytes, memoryview · Python database notes

Image, Visualization: PIL · Matplotlib, pylab · seaborn · bokeh · plotly

Tasky: Concurrency (threads, processes, more) · joblib · pty and pexpect

Stringy: strings, unicode, encodings · regexp · command line argument parsing · XML

date and time


speed, memory, debugging, profiling · Python extensions · semi-sorted

functools notes

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.


Setting the process name

It's OS-specific stuff, so there is no short portable way.

The easiest method is to install/use the setproctitle module - it aims to be portable and try its best on various platforms. Example use:

    import setproctitle
    setproctitle.setproctitle( os.path.basename(sys.argv[0]) )
except ImportError:

The above often means 'use the filesystem name that was used to run this' - but not always, so a hardcoded string can make sense.

Temporary files

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Python's standard library module tempfile has a number of choices:

creates a temporary file
returns (open_file_descriptor, abspath)
the open file handle is fairly secure in the sense that others cannot easily get that handle(verify)
but if you only wanted a unique file *name* you wouldn't care about that
you can ask it to add a a prefix and/or suffix (both default to none)
you can ask for it to be placed in a specific directory (defaults to system default)
you are responsible for cleanup
(if you want that to be automatic, look to TemporaryFile)
if the OS implements O_EXCL, that file is accessible only for the creating user (plus admins(verify))
creates a temporary directory
with similar 'only accessible for creating user'
returns the abspath
you are responsible for cleanup

More help, and context managers

  • TemporaryFile[3]
acts much like mkstemp
...but returns file-like object (rather than file handle)
that is destroyed on close / gc
depending on OS/filesystem, the directory entry is either not created at all, or removed immediately after creation
helps security a little (but not hard guarantee)
implies the filesystem will clean up the backing data only after you close (what behind the scenes is still) the file handle

  • NamedTemporaryFile[4]
like TemporaryFile but does specifically have a directory entry
its filename is in .name

  • SpooledTemporaryFile[5]
like TemporaryFile, but makes an attempt to buffer contents in memory a bit longer

  • TemporaryDirectory[6]
acts much like mkstemp
returns a stringy object that can be used as a context manager
removes contents and directory (fundamentally best-effort - there are ways to break that)

These are all usable as context managers, so instead of e.g.:

tmp = tempfile.NamedTemporaryFile()

you can write:

with tempfile.NamedTemporaryFile() as tmp:


  • since more is handled for you, there are some more hidden edge cases, and hidden platform-specific details

  • gettempdir() - finds the directory we put temporary files in (basically what is used if you supply dir=None to the above)

Temporary files are created in tempfile.gettempdir() unless you

hand in dir= to every call
set tempfile.tempdir to a default directory you want the module to use (has some details, so not necessarily recomended)

array, deque

A list can be used as a sort of deque, like:

append(val)     # insert on right side 
pop()           # take from right side
insert(0,val)   # insert on left side
pop(0)          # take from left side

However, list is primarily efficient for stack-like use - list.pop(0) and list.insert(0, val) are O(n) operations,

...while those are O(1) operations on collections.deque (added in 2.4). deque also has appendleft(), extendleft(), and popleft(), and some others that can be convenient and/or more readable.

You may also wish to know about the queue nodule, a multi-producer, multi-consumer, thread-safe queue.

See also:

Some library notes

pytest notes

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

Pytest wants to find tests itself - this is an inversion of control thing, with the point is that no matter how complex tests get, pytest can do everything for us without us having to hook it in specifically.

What files does pytest pick up to be tested?

  • the filenames specified
  • if none specified:
filenames named like test_*.py or * on the directory tree under the current dir(verify)
You can control this discovery, see e.g.

How does pytest decide what to run as tests?

  • functions prefixed test at module scope (verify)

  • classes prefixed Test, and then functions prefixed test inside them
...but only if that class does not have a constructor (__init__).
These classes are not intended to be classes with state (and it does not actually instantiate the class(verify))
just to collect functions (and potentially pollute a namespace less).
  • classes subclassed from unittest.TestCase (see unittest)

  • by marker, e.g. pytest -m slow picks up things decorated with @pytest.mark.slow
useful to define groups of tests, and run specific subsets

What does pytest actually consider success/failure?

Roughly: each test function will be a

  • success:
if it returns, AND
if all asserts contained (if any) are successful
  • failure: on the first failing assert
  • failure: on the first exception

There are "assert for me" functions, including:

unittest.assertEqual(a, b)
unittest.assertNotEqual(a, b)
unittest.assertIs(a, b)
unittest.assertIsNot(a, b)
unittest.assertIn(a, b)
unittest.assertNotIn(a, b)
unittest.assertIsInstance(a, b)
unittest.assertNotIsInstance(a, b)

...but many of those are shorter to write in your own assert

How do I test that something throws an exception (or does a warning)?

Has a few alternatives, also varying a little with whether you're testing that it should raise an error or that it doesn't.

The context manager form seems a brief-and-more-flexible way to filter for a specific error type and specific error text:

with pytest.raises(ValueError, match=r'.*found after.*'):
    # code that whines about some value parsing

You could also e.g. catch the specific error as you normally would. And if you need your test to fail in response, use use

except ZeroDivisionError as exc:

For warnings, pytest.warns can be used as a context manager that works much the same,

and the context object variant is probably easiest here
note: warnings.warn() by default emits a UserWarning - see
with pytest.warns(UserWarning, match=r'.*deprecated.*'):  #

Showing details

Pytest will try to give useful errors for failed tests, e.g. picking up the values that didn't compare as you wanted:

comparing long strings: a context diff is shown
comparing long sequences: first failing indices
comparing dicts: different entries

This can be customized, which is sometimes worth it to get more useful output from pytest runs.

On fixtures/mocking

Benchmarking,_performance_testing,_load_testing,_stress_testing,_etc.#Mocking.2C_monkey_patching.2C_fixtures Fixtures create reusable state/helpers for tests, and are great if you use the same data/objects.

In pytest, they are functions that are called before your function.

pytest has a few different things you could call fixtures.

Some given fixtures

Having a test function with a specifically named keyword, you get in some extra behaviour when pytest runs this test. Consider:

def test_download_to_file( tmp_path ):
    tofile_path = tmp_path / "testfile"  # this syntax works because tmp_path is a pathlib.Path object
    download('', tofile_path=tofile_path)
    assert os.path.exists( tofile_path )

tmp_path means "create directory for you, hand it in for you to use, and we will clean it up afterwards", which is a great helper you would otherwise have to write yourself (and test in itself).

For some other given fixtures, see e.g.

There is also @pytest.fixture decorator, which marks a function as a fixture To steal an example from [7], consider:

import pytest

def hello():
    return 'hello'

def world():
    return 'world'

def test_hello(hello, world):
    assert "hello world" == hello + ' ' + world

To keep this the first example short, this is only remembering some values for us and handing them into functions. Which is fairly pointless.

In the real world this is probably mostly useful more useful for setup and teardown.

Consider an example from [8]

def app_without_notes():
    app = NotesApp()
    return app

def app_with_notes():
    app = NotesApp()
    app.add_note("Test note 1")
    app.add_note("Test note 2")
    return app

...which comes from a basic "soooo I spend the first lines of every test just instantiating my application, can't I move that out?"

Doing teardown seems to be done by using a generator (this is a little creative syntax-wise, but lets pytest does most of the work for you)

def app_with_notes(app):
    app.add_note("Test note 1")
    app.add_note("Test note 2")
    yield app # state handed to the test
    app.notes_list = [] # clean up test's data

See also

On coverage

To do coverage checking at all, add


This mostly gets you a summary.

To see which lines aren't covered, read

If you want it to generate a browsable set of HTML pages, try:

--cov-report html:coverage-report





Win32 interface

pywin32, previously known as win32all, provides hooks into various parts of windows. Apparently with central module win32api. (see also its help file)

downloadable from sourceforge and with a homepage here.

Some code for this:


This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.










Creating ZIP files (in-memory)

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

We can use in-memory data and StringIO objects here - so this is sometimes simpler than doing the same with tarfile.

You can add content with either:

  • ZipFile.write - takes filename it will open - useful when you just wanted more control over what files to add
  • ZipFile.writestr takes a filename/ZipInfo and a bytestring - the filesystem isn't touched, which is probably what you want when adding in-memory data to in-memory zip files.
# Just an example snippet
z = zipfile.ZipFile(zip_sio, "w", zipfile.ZIP_DEFLATED) # or another compression level

for filename,filedata in (('filename1.txt', 'foo'),
                          ('filename2.txt', 'bar')):
return zip_sio.getvalue()


There are a few implementations/modules, including:

  • fftpack: used by numpy.fft, scipy.fftpack

Speed-wise: FFTW is faster, numpy is slower, scipy is slower yet (not sure why the np/sp difference when they use the same code) Think a factor 2 or 3 (potentially), though small cases can be drowned in overhead anyway.

Also, Optimized_number_crunching#On_plans_and_wisdom matters.

It does matter how the coupling works - there for example are more and less direct (overhead-wise) ways of using FFTW.

TODO: figure out threading, MPI stuff.

See also:

Other extensions

Bytecode / resolve-related notes

  • .py - source text
  • .pyc - compiled bytecode
  • .pyo - compiled bytecode, optimized. Written when python is used with -O. The difference with pyc is currently usually negligible.
  • .pyd - a (windows) dll with some added conventions for importing
(and path-and-import wise, it acts exactly like the above, not as a linked library)
note: native code rather than bytecode
  • (.pyi files are unrelated)

All of the above are searched for by python itself.

Python it will generate pyc or pyo

when they are imported them (not when the modules are run directly)
...with some exceptions, e.g. when importing from an egg, or zip file it will not alter those
...which means it can, for speed reasons, be preferable to distribute those with pyc/pyo files in them

There is some talk about changing these, see e.g. PEP 488

See also:

pyi files

These are called stub files, and are part of type-checker tooling.

These should only be read for their function signatures. They are syntactically valid python but they should be considered metadata, and you should have no runtime behaviour of them.

IDEs have been known to store what they managed to infer in these files. (and I looked them up because I wanted to know the reason vscode often doesn't show me the actual code when I ask for definition (F12 / Ctrl-click). This seems to have been broken / half-fixed in ways only some people understand for a few years now)

They are metnioned in Type hints PEP ( PEP-484)



documentation generators

This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

docstring formats

Not standardized. Options include:

now more common
see also PEP 287

Further notes

!=None versus is not None


  • 99% of the time it makes no difference
  • ...but in some cases it does. If you want a singular good habit that covers those cases too, it's is


PEP8 says that comparisons to singletons should be done with is resp. is not, not with equality (==).

Part of the reason is roughly that a class is free to implement the equality operator (==) any way it wants, whereas is is defined by the language and will always do what you think.

Another is that you may not have perfect overview of what coercion does

consider that 1 == True but 1 is not True.

Whenever a class doesn't redefine the equality operator, then == is often just as good.

Even when it does redefine it, it is still not necessarily a problem, due to what people usually use the equality comparison operator for.

Still, in general practice, is is cleaner.

One real-world example is numpy

Where you write

if ary==None:

What you probably wanted to check whether the variable has an array (or any object) assigned at all.

But numpy redefines the == comparison so that you can e.g. compare an array to an array, and an array to a scalar, evaluated element-wise -- and get an ndarray of bools, rather than a single python bool:

>>> numpy.array( [(1, 2), (2, 1)]  ) == 2
array([[False,  True],
       [ True, False]])

And ary==None would almost always give an all-False matrix (...because numpy won't even be able to store None unless you use dtype=object, which is often specifically avoided in number crunching if you can).


This article/section is a stub — some half-sorted notes, not necessarily checked, not necessarily correct. Feel free to ignore, or tell me about it.

(We are skipping the "never use globals" topic here)


You can always read variables from global scope (and other outer scopes)(verify).

When you want to assign, how would you know what to, though?

Would it be local scope unless there happened to be something called that in any outer/global scope? That's a little too flimsy and contextual

So python insists you make yourself clear.

Until told olderwise, variable writes go to local scope.

So if you want to write to global scope, you have to say so:

global nameatglobalcope

As this FAQ notes, it may be a little surprising that you get to read all globals.

One argument is that yes, Maybe reaching into other scopes would be clearer when always explicit, but remember that things like imported modules and their functions, built-ins, and more, are all effectively global, so you would be doing it a lot, and it may be more useful to only have to use global when you are doing something unusual so that it has specific debugging value.

You can argue that even if you only read, you can still use global to indicate you are accessing globals. Not because it does anything, but because it communicates to others you are mixing scopes. (eager linters consider this meh, though)

At a more technical level, global has two possible effects: (verify)

  • if the variable did not exist in global scope, it now declares it there, (and binds it in your more local scope as it always would)
  • if the variable already existed in global scope, it just binds it in your more local scope


If you understand the need for global, you may have also run into cases where that is crude.

Python 3 introduced nonlocal, which binds to the nearest containing scope that has the.

This makes sense e.g. for inner functions accessing e.g. the outer scope they were defined in, a case in which the global would be too crude an instrument

dunder methods