assert for exception parameters

[techtonik]

BPO	11759
Nosy	@ezio-melotti, @bitdancer, @durban

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2011-04-04.13:45:22.825>
created_at = <Date 2011-04-04.06:49:13.570>
labels = ['invalid', 'tests']
title = 'assert for exception parameters'
updated_at = <Date 2011-04-04.14:27:03.934>
user = 'https://bugs.python.org/techtonik'

bugs.python.org fields:

activity = <Date 2011-04-04.14:27:03.934>
actor = 'techtonik'
assignee = 'none'
closed = True
closed_date = <Date 2011-04-04.13:45:22.825>
closer = 'r.david.murray'
components = ['Tests']
creation = <Date 2011-04-04.06:49:13.570>
creator = 'techtonik'
dependencies = []
files = []
hgrepos = []
issue_num = 11759
keywords = []
message_count = 9.0
messages = ['132919', '132923', '132924', '132925', '132944', '132945', '132946', '132947', '132949']
nosy_count = 4.0
nosy_names = ['techtonik', 'ezio.melotti', 'r.david.murray', 'daniel.urban']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue11759'
versions = []

[techtonik]

I've just realized that unittest doesn't provide a way to test arguments of exception thrown during assertRaises check.

[durban]

What about this:

>>> class MyTestCase(TestCase):
...     def test_foo(self):
...             with self.assertRaises(SyntaxError) as cm:
...                     compile('asdf jkl', 'file.py', 'eval')
...             self.assertEqual('file.py', cm.exception.filename)

This isn't good enough?

[techtonik]

Looks like a hack (and not the obvious one). I guess no asserts return values like this, so that usage is not really intuitive for me.

[techtonik]

I found that successful assert in twisted returns exception object. But thanks for workaround anyway.

[bitdancer]

Using assertRaises as a context manager is not a hack, and is the correct way to do this in unittest.

[ezio-melotti]

See also bpo-6275.

[techtonik]

I've also found bpo-9587. Now I wonder how many people requested return vs context, and how many would vote for one vs another. Nothing personal - just a pure curiosity.

[ezio-melotti]

That was a design decision made by Guido (iirc), and even if it might seem not obvious at first, it's probably just because not everyone is Dutch.
Many people would probably vote on sequence.join(sep) too, but it's unlikely that another way to do it will be introduced.

The doc for assertRaises0 also is quite clear and even includes an example where the exception args are checked.

[techtonik]

Thanks for clarification, Ezio. I am still using Python 2.7 - that's why I've missed this part.