Specialize the regex type re.Pattern-CodePudding

Specializing the type of re.Pattern to re.Pattern[bytes], mypy correctly detects the type error:

import re
REGEX: re.Pattern[bytes] = re.compile(b"\xab.{2}")
def check(pattern: str) -> bool:
    if str == "xyz":
        return REGEX.fullmatch(pattern) is not None
    return True
print(check("abcd"))

Type mismatch detected:

$ mypy ~/main.py 
/home/oren/main.py:5: error: Argument 1 to "fullmatch" of "Pattern" has incompatible type "str"; expected "bytes"
Found 1 error in 1 file (checked 1 source file)

However, when I try to actually run the code I get a weird (?) message:

$ python ~/main.py
Traceback (most recent call last):
  File "/home/oren/main.py", line 2, in <module>
    REGEX: re.Pattern[bytes] = re.compile(b"\xab.{2}")
TypeError: 'type' object is not subscriptable

How come the type annotation bothers Python?

CodePudding user response：

The ability to specialize the generic re.Pattern and re.Match types using [str] or [bytes] was added in Python 3.9. It seems you are using an older Python version.

For Python versions earlier than 3.8 the typing module provides a typing.re namespace which contains replacement types for this purpose.

Since Python 3.8, they are directly available in the typing module and the typing.re namespace is deprecated (will be removed in Python 3.12).

Reference: https://docs.python.org/3/library/typing.html#typing.Pattern

Summary:

for Python <3.8, use typing.re.Pattern[bytes]
for Python 3.8, use typing.Pattern[bytes]
for Python 3.9 , use re.Pattern[bytes]

CodePudding user response：

Did you try to use typing module? I think the problem here occurs because re.Pattern[bytes] expression cannot be used like you want to. Try something like typing.re.Pattern[bytes].

I checked it on python3.7 and it works