better error messages for regular expression validation

[wbolster]

string fields can have a pattern for validation, and since re.search() is used under the hood, the pattern can be either a regular expression as a string or a compiled regular expression (re.compile(...) is cached and can have flags).

so far so good, but the error message uses this:

        "pattern": "Must match the pattern /{pattern}/.",

the str() conversion for compiled regexes looks like code:

>>> my_regex = re.compile('foo')

>>> f"oops: {my_regex}"
"oops: re.compile('foo')"

but the .pattern attribute is a string:

>>> f"oops: {my_regex.pattern}"
'oops: foo'

so, perhaps the error message for regular expressions could be improved. 🌳

[tomchristie]

Right now the pattern attribute is strictly a string...

https://github.com/encode/typesystem/blob/master/typesystem/fields.py#L106

I'd be very happy to:

• Accept a PR that switched that to allowing either a string or a regex.
• Added a pattern_regex property on the String field, which is used to store the compiled pattern.
• Switch the validation to use the compiled pattern_regex to perform the matching.

So I think __init__ would look something like this...

if pattern is None:
    self.patten is None
    self.pattern_regex is None
elif isinstance(pattern, str):
    self.pattern = pattern
    self.pattern_regex = re.compile(pattern)
elif isinstance(pattern, re.Pattern):
    self.pattern = pattern.pattern
    self.pattern_regex = pattern

[tomchristie]

Similar to this we'll also want beter support for customizing error messages, so that eg...

# Match valid variable names like 'someFunctionName123'
variable_name = typesystem.String(
    pattern="^a-zA-Z_[a-zA-Z0-9_]*$",
    errors={"pattern": "Must be a valid variable name."}
)

But we'll handle that under #28

[wbolster]

i'll cook a pr. it's gonna be simpler than your suggestions.

[tomchristie]

Sure thing 👍
However you implement, you'll want to make sure that the pattern attribute always gets set to a plain string - we inspect that attribute when generating JSON schema documents.