Generating mixed value for enum.Flag causes pretty-printer exception

[bartoszflis-silvair]

A self-contained reproduction:

import enum

from hypothesis import example, given
from hypothesis import strategies as st


class MyFlag(enum.Flag):
    ONE = 1
    TWO = 2


@given(st.sampled_from(MyFlag))
@example(MyFlag.ONE | MyFlag.TWO)
def test_pretty_printing(flag: MyFlag):
    assert isinstance(flag, MyFlag)

Tests fail with the following error message:

test_hypothesis_bug.py::test_pretty_printing Failed: [undefined]TypeError: can only concatenate str (not "NoneType") to str
@given(st.sampled_from(MyFlag))
>   @example(MyFlag.ONE | MyFlag.TWO)

tests/unit/test_hypothesis_bug.py:13: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
.venv/lib/python3.10/site-packages/hypothesis/core.py:1056: in _raise_to_user
    raise the_error_hypothesis_found
.venv/lib/python3.10/site-packages/hypothesis/core.py:470: in execute_explicit_examples
    bits = ", ".join(nicerepr(x) for x in arguments) + ", ".join(
.venv/lib/python3.10/site-packages/hypothesis/core.py:471: in <genexpr>
    f"{k}={nicerepr(v)}" for k, v in example_kwargs.items()
.venv/lib/python3.10/site-packages/hypothesis/internal/reflection.py:452: in nicerepr
    return re.sub(r"(\[)~([A-Z][a-z]*\])", r"\g<1>\g<2>", pretty(v))
.venv/lib/python3.10/site-packages/hypothesis/vendor/pretty.py:101: in pretty
    printer.pretty(obj)
.venv/lib/python3.10/site-packages/hypothesis/vendor/pretty.py:199: in pretty
    return printer(obj, self, cycle)
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

obj = <MyFlag.TWO|ONE: 3>
p = <hypothesis.vendor.pretty.RepresentationPrinter object at 0x7fc633f41090>
cycle = False

    def _repr_enum(obj, p, cycle):
>       p.text(type(obj).__name__ + "." + obj.name)
E       TypeError: can only concatenate str (not "NoneType") to str

.venv/lib/python3.10/site-packages/hypothesis/vendor/pretty.py:804: TypeError

Root cause: mixed-value flags don't have a name.

Python 3.10
hypothesis 6.82.0

[Zac-HD]

Thanks for the report! This is a neat little bug, fortunately the solution should be pretty self-contained in that _repr_enum() function for whoever gets to it first.

[bartoszflis-silvair]

Thank you for fixing this!