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printf-string-formatting (UP031)

Derived from the pyupgrade linter.

Fix is sometimes available.

What it does

Checks for printf-style string formatting, and offers to replace it with str.format calls.

Why is this bad?

printf-style string formatting has a number of quirks, and leads to less readable code than using str.format calls or f-strings. In general, prefer the newer str.format and f-strings constructs over printf-style string formatting.

Example

"%s, %s" % ("Hello", "World")  # "Hello, World"

Use instead:

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

f"{'Hello'}, {'World'}"  # "Hello, World"

Fix safety

In cases where the format string contains a single generic format specifier (e.g. %s), and the right-hand side is an ambiguous expression, we cannot offer a safe fix.

For example, given:

"%s" % val

val could be a single-element tuple, or a single value (not contained in a tuple). Both of these would resolve to the same formatted string when using printf-style formatting, but resolve differently when using f-strings:

val = 1
print("%s" % val)  # "1"
print("{}".format(val))  # "1"

val = (1,)
print("%s" % val)  # "1"
print("{}".format(val))  # "(1,)"

References