Pytest: `pytest.raises` seems to let the exception propagate.

[AvivC]

with pytest.raises(ValueError):
    client.get('/endpoint')

This code tests a Flask application.
The ValueError is not caught, and eventually crashes the test.
Any idea what the problem could be?

[codewarrior0]

Just a hunch: Earlier in your test suite, you have code like except TypeError, ValueError:

This is the old-style except syntax kicking in, with the effect of writing except TypeError as ValueError, which changes the name ValueError to an instance of TypeError. The correct code for catching multiple exception types is except (TypeError, ValueError):

[RonnyPfannschmidt]

Please paste/attach a full output

[AvivC]

As you can see, it outputs Failed: DID NOT RAISE. But the ValueError propagates and crashes the test.

←[1m        with pytest.raises(ValueError):←[0m
←[1m>           client.get('/tuple5')←[0m
←[1m←[31mE           Failed: DID NOT RAISE←[0m

tests\test_basic.py:977: Failed
---------------------------- Captured stderr call -----------------------------
[2016-01-02 16:15:58,851] ERROR in app: Exception on /tuple5 [GET]
Traceback (most recent call last):
  File "c:\users\owner\desktop\flask\flask\app.py", line 1994, in wsgi_app
    response = self.full_dispatch_request()
  File "c:\users\owner\desktop\flask\flask\app.py", line 1621, in full_dispatch_
request
    response = self.make_response(rv)
  File "c:\users\owner\desktop\flask\flask\app.py", line 1719, in make_response
    return self._make_response_from_tuple(view_func_rv)
  File "c:\users\owner\desktop\flask\flask\app.py", line 1724, in _make_response
_from_tuple
    response_body, status, headers = self._extract_response_parts_from_tuple(ret
val_from_view_func)
  File "c:\users\owner\desktop\flask\flask\app.py", line 1738, in _extract_respo
nse_parts_from_tuple
    raise ValueError('Tuples returned from view functions must be composed as on
e '
ValueError: Tuples returned from view functions must be composed as one of the f
ollowing options: (response_body, status, headers), (response_body, status) or (
response_body, headers).
←[1m←[31m============== 1 failed, 257 passed, 11 skipped in 7.67 seconds =======
========←[0m

[RonnyPfannschmidt]

Thats normal test client behaviour

It makes a 500 response

Unless you configure it to pass trough exceptions

[AvivC]

Sorry I don't understand what you mean.
client.get('/endpoint') raises a ValueError. I assume pytest.raises(ValueError) should catch the error and pass the test. However it does two weird things:

1. Failes the test - says that the ValueError was not raised (Failed: DID NOT RAISE).
2. And (!) let the ValueError propagate and crash the test.

How is this possible? And how can I simply assert that the exception is raised (this is what I'm testing for).

[astraw38]

It looks like the exception is being logged to stderr, not being raised at all. Try it manually via:

try:
     client.get('/endpoint')
except ValueError:
     pass
else:
     raise Exception("Value error not found")

And you should see the same result, no?

[RonnyPfannschmidt]

The test client logs the exception and returns a response with the 500 status

Please configure the test client to pass trough exceptions when you want to catch them

[RonnyPfannschmidt]

Closing as invalid, in the PR where he experienced the issue he already fixed his api misuse

[davidthornton]

Could you link to that pull request? Or outline how to configure the test client to pass through exceptions?