General

nonlocal: allow modification of outer function variable from inner function
- Otherwise, inner function can access outer function variables, but cannot modify them.
*args, **kwargs in function arguments
- *args: collect positional arguments as a list
- **kwargs: collect arguments as a dictionary
Unit type in Python:
- Ellipsis with value ... or Ellipsis
- NoneType with value None
Walrus operator (:=): assignment that is also an expression
Set maximum number of recursive calls: sys.setrecursionlimit() ᵈ
Check if a list l is empty: "empty" if not l else "non-empty"
Concatenate two lists: l1 + l2
functools.singledespatch: used to have something like function overloading of C++
- Same function but definition is chosen based on type of argument
- Register different definitions of function with functools.register

`sort()` vs `sorted()`

list.sort():
- only for list
- modifies list in place
sorted():
- for any iterable
- input is not modified

https://docs.python.org/3/howto/sorting.html

Positional only argument syntax (PEP 570)

def foo(pos_only_args, /, pos-or-kw-args, *, kw-only-args):
    body

Ellipsis

A type with only a single possible value.

>>> type(Ellipsis)
<class 'ellipsis'>

>>> type(...)
<class 'ellipsis'>

>>> Ellipsis: Ellipsis; print("OK")
OK

>>> Ellipsis: ellipsis
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ellipsis' is not defined

>>> type(ellipsis)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'ellipsis' is not defined

I guess ellipsis is something not meant to be accessed by users.

re (regular expression)

https://docs.python.org/3/library/re.html

>>> c = re.compile("\w\da")
>>> c.match("a0abbb")
<re.Match object; span=(0, 3), match='a0a'>

Syntax

., ^, $, *, +, ?
{m}, {m,}, {m,n}, {,m}
[], |, (..)
\NUM: refer to a group
\b: ??
\B: ??
\d: [0-9]
\D: negation of \d
\s: whitespaces ([\t\n\f\v ] and more)
\S: negation of \s
\w: [0-9A-Za-z_]
\W: negation of \w
\A: match start of the string ??
\Z: match end of the string ??

—

*?, +?, ??, {m}+ (non-greedy / minimal version)
*+, ++, ?+ (possessive quantifiers ??)

enum types

import enum

class Op(enum.Enum):
    ADD = 1
    SUBTRACT = 2

Structural pattern matching (`match` & `case`)

https://peps.python.org/pep-0636/* mypy https://docs.python.org/3/library/typing.html

# type: ignore: skip type checking for line with this at the end
Type variable: A = TypeVar("A")
Any type: Any

numpy support type annotations too: https://numpy.org/doc/stable/reference/typing.html

import numpy.typing as npt
nptyping: Third party library for better type annotations for numpy
- https://github.com/ramonhagenaars/nptyping

pdb

Python debugger part of stdlib: https://docs.python.org/3/library/pdb.html

. is current line.
Frame

Commands

l: show current position in code
- l <line-num>
n: next command
b: set break point
s: next step
p: print value
a: print arguments of current function as a list
pp: same as p but with pretty-printing
w: where
r: continue execution till current function returns
q: abort and program being debugged and exit debugger
whatis: print type of an expression
source: attempt to print the source code of an object
restart: restart program being debugged
run: start debugging the program
display <expr>: show a message when the argument expression changes
interact: start an interpreter at this point
alias: create an alias for a command
unalias: delete an alias

Drop to pdb during execution

import pdb
pdb.set_trace()

The builtin breakpoint() available from 3.7 onwards, does the same thing. https://docs.python.org/3/library/functions.html#breakpoint

.pdbrc

https://stackoverflow.com/questions/5169702/how-do-i-list-the-current-line-in-python-pdb

alias ll u;;d;;l