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format-literals (UP030)

Derived from the pyupgrade linter.

Fix is sometimes available.

What it does

Checks for unnecessary positional indices in format strings.

Why is this bad?

In Python 3.1 and later, format strings can use implicit positional references. For example, "{0}, {1}".format("Hello", "World") can be rewritten as "{}, {}".format("Hello", "World").

If the positional indices appear exactly in-order, they can be omitted in favor of automatic indices to improve readability.

Example

"{0}, {1}".format("Hello", "World")  # "Hello, World"

Use instead:

"{}, {}".format("Hello", "World")  # "Hello, World"

This fix is marked as unsafe because:

  • Comments attached to arguments are not moved, which can cause comments to mismatch the actual arguments.
  • If arguments have side effects (e.g., print), reordering may change program behavior.

References