What is the purpose of the single underscore variable in Python

Question

What is the meaning of   after for in this code   if tbh bag     n   0    for   in tbh bag atom set          n    1

User · Answer

As far as the Python languages is concerned, _ has no special meaning. It is a valid identifier just like _foo, foo_ or _f_o_o_.

Any special meaning of _ is purely by convention. Several cases are common:

A dummy name when a variable is not intended to be used, but a name is required by syntax/semantics.

# iteration disregarding content
sum(1 for _ in some_iterable)
# unpacking disregarding specific elements
head, *_ = values
# function disregarding its argument
def callback(_): return True

Many REPLs/shells store the result of the last top-level expression to builtins._.

The special identifier _ is used in the interactive interpreter to store the result of the last evaluation; it is stored in the builtins module. When not in interactive mode, _ has no special meaning and is not defined. [source]

Due to the way names are looked up, unless shadowed by a global or local _ definition the bare _ refers to builtins._ .
```
>>> 42
42
>>> f'the last answer is {_}'
'the last answer is 42'
>>> _
'the last answer is 42'
>>> _ = 4  # shadow ``builtins._`` with global ``_``
>>> 23
23
>>> _
4
```
Note: Some shells such as ipython do not assign to builtins._ but special-case _.
In the context internationalization and localization, _ is used as an alias for the primary translation function.

gettext.gettext(message)

Return the localized translation of message, based on the current global domain, language, and locale directory. This function is usually aliased as _() in the local namespace (see examples below).

User · Answer

Underscore   is considered as  I don t Care  or  Throwaway  variable in Python   The python interpreter stores the last expression value to the special variable called      gt  gt  gt  10  10   gt  gt  gt     10   gt  gt  gt      3  30  The underscore   is also used for ignoring the specific values  If you don   t need the specific values or the values are not used  just assign the values to underscore   Ignore a value when unpacking  x     y    1  2  3    gt  gt  gt  x 1   gt  gt  gt  y  3   Ignore the index  for   in range 10            do something

User · Answer

has 3 main conventional uses in Python   To hold the result of the last executed expression  statement  in an interactive interpreter session  see docs   This precedent was set by the standard CPython interpreter  and other interpreters have followed suit  For translation lookup in i18n  see the gettext documentation for example   as in code like raise forms ValidationError    quot Please enter a correct username quot      As a general purpose  quot throwaway quot  variable name   To indicate that part of a function result is being deliberately ignored  Conceptually  it is being discarded    as in code like  label  has label      text partition        As part of a function definition  using either def or lambda   where the signature is fixed  e g  by a callback or parent class API   but this particular function implementation doesn t need all of the parameters  as in code like  def callback         return True   For a long time this answer didn t list this use case  but it came up often enough  as noted here  to be worth listing explicitly     This use case can conflict with the translation lookup use case  so it is necessary to avoid using   as a throwaway variable in any code block that also uses it for i18n translation  many folks prefer a double-underscore      as their throwaway variable for exactly this reason   Linters often recognize this use case  For example year  month  day   date   will raise a lint warning if day is not used later in the code  The fix  if day is truly not needed  is to write year  month      date    Same with lambda functions  lambda arg  1 0 creates a function requiring one argument but not using it  which will be caught by lint  The fix is to write lambda    1 0  An unused variable is often hiding a bug typo  e g  set day but use dya in the next line

User · Answer

It s just a variable name  and it s conventional in python to use   for throwaway variables   It just indicates that the loop variable isn t actually used

User · Answer

There are 5 cases for using the underscore in Python   For storing the value of last expression in interpreter   For ignoring the specific values   so-called    I don   t care      To give special meanings and functions to name of variables or functions   To use as    internationalization  i18n     or    localization  l10n     functions   To separate the digits of number literal value    Here is a nice article with examples by mingrammer

[python] What is the purpose of the single underscore "_" variable in Python?

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