Explicit support for special-use domains.

[jenstroeger]

Hypothesis’ emails() strategy generated an email address 0@a.arpa which was rejected by the email-validator package:

>>> import email_validator
>>> email_validator.validate_email('0@a.arpa')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/.../.venv/lib/python3.9/site-packages/email_validator/__init__.py", line 319, in validate_email
    domain_part_info = validate_email_domain_part(parts[1], test_environment=test_environment, globally_deliverable=globally_deliverable)
  File "/.../.venv/lib/python3.9/site-packages/email_validator/__init__.py", line 580, in validate_email_domain_part
    raise EmailSyntaxError("The domain name %s is a special-use or reserved name that cannot be used with email." % domain_i18n)
email_validator.EmailSyntaxError: The domain name a.arpa is a special-use or reserved name that cannot be used with email.

The email-validator package defines “special-use or reserved names” here.

Now, Hypothesis builds email addresses using the internal domains() strategy:

hypothesis/hypothesis-python/src/hypothesis/strategies/_internal/core.py

Lines 2029 to 2031 in 48165d9

	return builds("{}@{}".format, local_part, domains()).filter(
	lambda addr: len(addr) <= 254
	)

which doesn’t have explicit support to filter out or use these special-use domain names.

I think adding an optional kwarg allow_special_use=True to the emails() strategy which then propagates to the DomainNameStrategy class would be useful. Happy to supply a PR?

[Zac-HD]

A PR would be great, thanks! Let's add allow_special_use: bool = True to provisional.domains(), and *, domains: st.SearchStrategy[str] = domains() to st.emails()¹ - that way people can also pass e.g. domains=st.just("employer.com"), so it's a more general upgrade 🙂

Footnotes

1. My reading of RFC-5322 is that special domains are fine according to the standard so IMO Hypothesis should include them by default, and the email-validator docs describes how to enable them in testing, noting that they're off by default as an anti-footgun. ↩