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

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.

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

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       gt  gt  gt  42 42  gt  gt  gt  f the last answer is       the last answer is 42   gt  gt  gt     the last answer is 42   gt  gt  gt      4    shadow   builtins     with global        gt  gt  gt  23 23  gt  gt  gt    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

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

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

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