What Does the Single Underscore '_' Mean in Python?

Meaning of the Single Underscore Variable in Python

In Python, the single underscore symbol ("_") plays a special role in specific contexts, carrying conventional meanings for different purposes.

Conventional Uses of "_"

1. Result Placeholder in Interactive Interpreter:

   • "_" stores the result of the last executed expression in an interactive Python session, providing easy access to the previous output.

2. Translation Lookup:

   • "_" indicates a placeholder for translation during internationalization (i18n) processes. It is used to retrieve translated strings, ensuring proper language localization.

3. "Throwaway" Variable:

   • "-" serves as a placeholder to deliberately discard a section of a function's result or unused parameters in a signature. This practice helps keep code concise and avoids unnecessary variable assignment.

Example:

Discarding a Variable

label, has_label, _ = text.partition(':')

Here, "_" is used to ignore the third return value from the text.partition() function, which is the index of the separator character.

Unused Function Parameter

def callback(_):
    return True

In this example, the lambda function callback takes one parameter, "_," but does not use it, as indicated by the single underscore.

Note: The third usage of "_" as a "throwaway" variable can conflict with its use for translation lookup. Therefore, a double underscore ("__") is commonly used as an alternative "throwaway" variable in scenarios that involve translation.

Linters often recognize the use of "_" as a "throwaway" variable and raise warnings if the variable is unused. Additionally, in pattern matching statements introduced in Python 3.10, "_" serves as a wildcard pattern, indicating that the runtime should ignore the value associated with that variable.

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