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

*args and **kwargs

• *args: positional arguments
• *kwargs: keyword arguments

fstrings

Involves a mini-language.

• Literal { and } should be {{ and }} respectively.

—

Print variable name and value (useful for debugging):

>>> a=3
>>> f"{a=}"
'a=3'

>>> f"{a}"
'3'

—

• Padding

Sign (only for numbers):

z option is only from v3.11.

More: #, ,, _

—

Alignemnt:

>>> s="hello"
>>> f"{s:^10}"
'  hello   '

>>> f"{s:10}"
'hello     '

>>> f"{s}"
'hello'

>>> f"{s:*^11}"
'***hello***'

>>> f"{s:=10}"
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: '=' alignment not allowed in string format specifier

—

Conversion

—

See:

• https://docs.python.org/3/library/string.html#format-specification-mini-language
• https://docs.python.org/3/library/string.html#format-examples
• https://docs.python.org/3/library/string.html#formatstrings
• https://docs.python.org/3/reference/lexical_analysis.html#f-strings

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