Underline _ as variable name in Python (2023)

What is the purpose of each _ underscore variable in Python?

_has 3 main conventional uses in Python:

  1. To retain the result of the last executed expression in an interactive expression
    Interpretation session (see documents). This precedent was set by the CPython standard
    Interpreters and other interpreters followed

  2. For the i18n translation search (see the
    get texts
    documentation, for example) as in code as

    Generate forms.ValidationError(_("Please enter a correct username"))
  3. As a generic "disposable" variable name:

    1. to see this part
      of a function's result is intentionally ignored (conceptually discarded), as in code like:

      label, has_label, _ = text.partition(':')
    2. As part of a function definition (using anydefinitiveÖlambda), Wo
      the signature is fixed (e.g. via a callback or a main class API) but
      This specific function implementation doesn't need all of them
      Parameters, as in code like:

      return true

      [This answer hasn't listed this use case for a long time, but as noted here, it's come up often enough to be worth mentioning explicitly.]

    This use case may conflict with the translation search use case, so avoiding its use is necessary_as a disposable variable in any block of code that also uses it for i18n translation (many people prefer a double underscore,__, exactly for this reason as a disposable variable).

    Linters generally recognize this use case. For exampleyear, month, day = date()generates a lint warning whenMarkingit is not used later in the code. the solution, yesMarkingIt's not really necessary, it's writingyear, month, _ = date(). The same applies to lambda functions,Lambda-arg: 1,0Create a function that takes an argument but doesn't use it, which lint will capture. The solution is to writelambda_: 1,0. An unused variable usually hides a mistake/typo (e.g. settingMarkingbut i useMarkingon the next line).

    The pattern matching feature added in Python 3.10 increased this use of "convention" to "language syntax", wherePhosphorThe statements relate to: in cases of party,_is a wildcard pattern and the runtime doesn't even bind a value to the symbol in this case.

    Keep this in mind for other use cases._is still a valid variable name and therefore will continue to keep objects alive. In cases where this is not desired (for example, to free up disk space or external resources), afrom the nameCall will convince linters to use the namejQuickly delete the reference to the object.

What do the single and double underscores in front of an object name mean?

single underline

Within a class, names preceded by an underscore indicate to other programmers that the attribute or method is to be used within that class. but not privacyforcedIn any case.
The use of leading underscores for functions in a module indicates that it must not be imported from elsewhere.

From the PEP-8 style guide:

_single_main_underscore: weak indicator of "internal use". For example.from M import *Objects whose name begins with an underscore are not imported.

Double underscore (name destruction)

From the Python docs:

Any form identifier__Spam(at least two leading underscores, at most one trailing underscore) is textually replaced by_classname__spam, Woclass nameis the current class name without the leading underscores. This manipulation is done without regard to the syntactic position of the identifier, so it can be used to define instances of private classes and class variables, methods, variables stored in globals, and even variables stored in instances. private for this class on instances of other classes.

And a notice from the same site:

Name manipulation is intended to provide classes with an easy way to define "private" variables and instance methods without worrying about instance variables defined by derived classes or manipulation of instance variables via code outside the class. Note that the mutilation rules are primarily intended to prevent accidents;It is still possible for a given soul to access or change a variable that is considered private.


>>> Class MyClass():
...def __init__(self):
... self.__superprivate="Hola"
... self._semiprivate = ", ¡mundo!"
>>> mc = MiClase()
>>> Imprimir mc.__superprivate
Tracing (recent calls):
File "<stdin>", line 1, in <module>
AttributeError: myClass instance has no '__superprivate' attribute
>>> Print mc._semiprivate
, welt!
>>> print mc.__dict__
{'_MyClass__superprivate': 'Hola', '_semiprivate': ', ¡mundo!'}

Why use the underscore ('_') as a variable name?

When unpacking lists/tuples_It is normally used for values ​​that you don't need later. If you look closely at this code, the_The variable is actually not used anywhere.

Note that no Python REPL_refers to the last result.

>>> 2+2
>>> _

Is the underscore in Python variable names important to the interpreter?

  1. Single leading underscore: _var
    The underscore prefix is ​​a hint to another programmer that a variable or method beginning with a single underscore is for internal use. This convention is defined in PEP 8.

  2. single final underscore: var_
    Sometimes the most appropriate name for a variable is already occupied by a keyword. So names like class or def cannot be used as variable names in Python. In that case, you can add a single underscore to resolve the naming conflict.

  3. Double leading underscore: __var
    Things are a little different for Python class attributes (variables and methods) that start with double underscores.
    A double underscore prefix causes the Python interpreter to rewrite the attribute name to avoid naming conflicts in subclasses.
    This is also known as name manipulation: the interpreter changes the variable name so that collisions are more difficult when the class is later extended.

  4. Double underscore at start and end:eras
    Perhaps surprisingly, name handling does not apply when a name begins and ends with double underscores. Variables surrounded by a double underscore as a prefix and suffix are not affected by the Python interpreter.

These paragraphs are taken from https://dbader.org/. More information and examples can be found on the page.

Underscore after a variable name in Python

A trailing underscore has no semantics associated with it. AfterPEP 8, the style guide for Python, encourages users to use trailing underscores to avoid conflicts with Python keywords and/or Python internals:

single_trailing_underscore_: conventionally used to avoid conflicts with the Python keyword, for example

Tkinter.Toplevel(master, class_='ClassName')

To uselower_means the internal name for sets i.e.lower, it is not shaded during the function call or loses its known reference.

Lider Python underscore _variables

It's a naming convention for private variables. See 9.6, private variables: http://docs.python.org/tutorial/classes.html#private-variables

Underline _ like variable names in Python

E,_is a traditional name for "I don't care" (which unfortunately conflicts with its use on I18N, but that's a separate topic ;-). By the way, in today's Python:

_,s = min((length(values[s]), s)
for s in squares
silence(values[s]) > 1

could you code

s = min((s to s squared if len(values[s])>1),
key=lambda s: len(values[s]))

(I'm not sure which version of Python Peter wrote, but the expression he uses is an example of "decorate-sort-undecorate" [[DSU]], except with min instead of sort, and in today's Python theClave =The optional parameter is usually the best way to do DSU ;-).

Using underscores for variables that match keyword name in Python

As stated in https://pep8.org/#function-and-method-arguments

When a function argument name conflicts with a reserved keyword, it's often best to add a single underscore rather than using an abbreviation or misspelling. ThereforeClassroom_it's better than class. (It might be better to avoid such conflicts by using a synonym.)

Note that a leading underscore, for example_Classroomusually includes a private class attribute or method, like this:

MyClass Class:
def __init__(self):
self._my_private_variable = 1

to allow

More information at https://pep8.org/#descriptive-naming-styles

_single_main_underscore: weak indicator for “internal use”. For example.from M import *Objects whose name begins with an underscore are not imported.

single_trailing_underscore_: Used by convention to avoid conflicts
with the Python keyword

Underline variable using Vise operator in Python

you use the variablefictitiousto filter the string. Therefore, do not substitute_.

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Can we use _ as variable name in Python? ›

Rules for Python variables: A variable name must start with a letter or the underscore character. A variable name cannot start with a number. A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )

What does _ before a variable mean in Python? ›

The underscore prefix is meant as a hint to another programmer that a variable or method starting with a single underscore is intended for internal use. This convention is defined in PEP 8. This isn't enforced by Python. Python does not have strong distinctions between “private” and “public” variables like Java does.

What is the use of _ and __ in Python? ›

The names in the class, which have leading underscore (_) are for indicating to the other users that the attribute or function is intended to be private in the program. The users are recommended that they should use single underscore (_) for semiprivate and double underscore (__) for fully private variables.

Why use underscore in variable names Python? ›

The python interpreter stores the last expression value to the special variable called _ . 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.

Can you use _ in variable names? ›

Go variable naming rules: A variable name must start with a letter or an underscore character (_) A variable name cannot start with a digit.

Is _ a valid variable name? ›

Follow these rules when naming a symbolic variable: The first character must be one of the following: A-Z, (a-z), _, #, $, @.

What is _ in Python variable? ›

Single standalone underscore _ is a valid character for a Python identifier, so it can be used as a variable name. According to Python doc, the special identifier _ is used in the interactive interpreter to store the result of the last evaluation. It is stored in the builtin module. Here is an example.

What is _ in variable name? ›

The underscore in variable names is completely optional. Many programmers use it to differentiate private variables - so instance variables will typically have an underscore prepended to the name. This prevents confusion with local variables. Save this answer.

What is _ in Python method? ›

Python automatically stores the value of the last expression in the interpreter to a particular variable called "_." You can also assign these value to another variable if you want.

WHAT IS AN _ used for? ›

The underscore ( _ ) is also known as an understrike, underbar, or underline, and is a character that was originally on a typewriter keyboard and was used simply to underline words or numbers for emphasis. Today, the character is used to create visual spacing in a sequence of words where whitespace is not permitted.

What is _ in class Python? ›

In Python, the use of an underscore in a function name indicates that the function is intended for internal use and should not be called directly by users. It is a convention used to indicate that the function is "private" and not part of the public API of the module.

What is for _ in range Python? ›

When you are not interested in some values returned by a function we use underscore in place of variable name . Basically it means you are not interested in how many times the loop is run till now just that it should run some specific number of times overall.

What does _ before variable name mean? ›

A single leading underscore in front of a variable, a function, or a method name means that these objects are used internally. This is more of a syntax hint to the programmer and is not enforced by the Python interpreter which means that these objects can still be accessed in one way on another from another script.

Why do people use _ in file names? ›

Some applications and computer scripts may not recognize spaces or will process your files differently when using spaces. A best practice is to replace spaces in file names with an underline (_) or hyphen (-).

Can a variable name end with underscore? ›

Variable name may not start with a digit or underscore, and may not end with an underscore. Double underscores are not permitted in variable name.

What variable names to avoid in Python? ›

Names to Avoid

Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character variable names.

Is _ a valid variable name in C? ›

Names can contain letters, digits and underscores. Names must begin with a letter or an underscore (_) Names are case sensitive ( myVar and myvar are different variables)

Which symbol is not allowed in variable name? ›

The $ sign is not allowed as the initial character of a user-defined variable. The period, the underscore, and the characters $, #, and @ can be used within variable names. For example, A. _$@#1 is a valid variable name.

Is _ an identifier? ›

An identifier starts with a letter or underscore and consists of: Letters. Numbers. _ (underscore)

What does _ mean in for loop? ›

"_" means you won't need a name for a var that will not be used.

What is _ each ()? ›

Syntax. _.each(list, iteratee, [context]) each method iterates over a given list of element, call the iteratee function which is bound to context object, if passed. Iteratee is called with three parameters: (element, index, list). In case of JavaScript object, iteratee's object will be (value, key, list).

What is a _ called? ›

Updated: 12/31/2022 by Computer Hope. Alternatively known as a low line, low dash, and understrike, the underscore ( _ ) is a symbol found on the same keyboard key as the hyphen. The picture shows an example of an underscore at the beginning and end of the word "Underscore."

Why do people use _ instead of space? ›

Not all filesystems and applications historically deal very well with spaces in filenames. To avoid any such issues underscores are often substituted as they are semantically similar.

What is underscore comma in Python? ›

The _ in the Python shell also refers to the value of the last operation. Hence >>> 1 1 >>> _ 1. The commas refer to tuple unpacking. What happens is that the return value is a tuple, and so it is unpacked into the variables separated by commas, in the order of the tuple's elements.

What is _init _ in Python? ›

The __init__ method is the Python equivalent of the C++ constructor in an object-oriented approach. The __init__ function is called every time an object is created from a class. The __init__ method lets the class initialize the object's attributes and serves no other purpose. It is only used within classes.

What is import _ in Python? ›

In Python, you use the import keyword to make code in one module available in another. Imports in Python are important for structuring your code effectively. Using imports properly will make you more productive, allowing you to reuse code while keeping your projects maintainable.

What is __ name __ in Python? ›

The __name__ variable (two underscores before and after) is a special Python variable. It gets its value depending on how we execute the containing script. Sometimes you write a script with functions that might be useful in other scripts as well. In Python, you can import that script as a module in another script.

Can identifier end with _? ›

An identifier can be composed of letters (both uppercase and lowercase letters), digits and underscore '_' only.

What is _ variable in Python? ›

Single Post Underscore is used for naming your variables as Python Keywords and to avoid the clashes by adding an underscore at last of your variable name.

What Cannot be used as a variable name in Python? ›

We cannot use Python built-in function names as variables. Built-in function names cannot be used as variable names even though they are not part of Python reserved keywords list. For example, we should not use the word list as a variable name because it is a built-in function in Python.

Which variable names are illegal in Python? ›

Illegal Variable Names in Python
  • We cannot start a variable name with a dash (-). ...
  • Variable names cannot start with a number. ...
  • We cannot use space to separate words in a variable name. ...
  • We cannot use Python reserved keywords as variable names. ...
  • We cannot use Python built-in function names as variables.
Nov 11, 2021

What is _ in Python for loop? ›

Your question is which one of these is being used in the example in your code. The answer would be that is a throwaway variable (case 3), but its contents are printed here for debugging purposes. where _ immediately signals the reader that the value is not important and that the loop is just repeated 10 times.

What is _ used for? ›

The underscore ( _ ) is also known as an understrike, underbar, or underline, and is a character that was originally on a typewriter keyboard and was used simply to underline words or numbers for emphasis. Today, the character is used to create visual spacing in a sequence of words where whitespace is not permitted.

What symbols are not allowed in Python? ›

An identifier in Python cannot use any special symbols like !, @, #, $, % etc.

Which of these Cannot be used as a variable name? ›

You cannot use keywords like int , for , class , etc as variable name (or identifiers) as they are part of the Java programming language syntax.


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