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unspecified-encoding (PLW1514)

Derived from the Pylint linter.

Fix is always available.

This rule is unstable and in preview. The --preview flag is required for use.

What it does

Checks for uses of open and related calls without an explicit encoding argument.

Why is this bad?

Using open in text mode without an explicit encoding can lead to non-portable code, with differing behavior across platforms. While readers may assume that UTF-8 is the default encoding, in reality, the default is locale-specific.

Instead, consider using the encoding parameter to enforce a specific encoding. PEP 597 recommends the use of encoding="utf-8" as a default, and suggests that it may become the default in future versions of Python.

If a locale-specific encoding is intended, use encoding="locale" on Python 3.10 and later, or locale.getpreferredencoding() on earlier versions, to make the encoding explicit.

Fix safety

This fix is always unsafe and may change the program's behavior. It forces encoding="utf-8" as the default, regardless of the platform’s actual default encoding, which may cause UnicodeDecodeError on non-UTF-8 systems.

with open("test.txt") as f:
    print(f.read()) # before fix (on UTF-8 systems): 你好，世界！
with open("test.txt", encoding="utf-8") as f:
    print(f.read()) # after fix (on Windows): UnicodeDecodeError

Example

open("file.txt")

Use instead:

open("file.txt", encoding="utf-8")

References