Function call with * and generator hide exception raised by generator.

[falsetru]

BPO	11944
Nosy	@ncoghlan, @merwok, @durban, @vadmium
Superseder	bpo-4806: Function calls taking a generator as star argument can mask TypeErrors in the generator

^{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 2012-02-04.02:06:33.043>
created_at = <Date 2011-04-28.01:00:46.607>
labels = ['interpreter-core', 'type-bug']
title = 'Function call with * and generator hide exception raised by generator.'
updated_at = <Date 2012-02-04.02:06:33.042>
user = 'https://bugs.python.org/falsetru'

bugs.python.org fields:

activity = <Date 2012-02-04.02:06:33.042>
actor = 'terry.reedy'
assignee = 'none'
closed = True
closed_date = <Date 2012-02-04.02:06:33.043>
closer = 'terry.reedy'
components = ['Interpreter Core']
creation = <Date 2011-04-28.01:00:46.607>
creator = 'falsetru'
dependencies = []
files = []
hgrepos = []
issue_num = 11944
keywords = []
message_count = 4.0
messages = ['134632', '134636', '134665', '134712']
nosy_count = 5.0
nosy_names = ['ncoghlan', 'falsetru', 'eric.araujo', 'daniel.urban', 'martin.panter']
pr_nums = []
priority = 'normal'
resolution = 'duplicate'
stage = None
status = 'closed'
superseder = '4806'
type = 'behavior'
url = 'https://bugs.python.org/issue11944'
versions = ['Python 2.7', 'Python 3.2']

[falsetru]

Expected "TypeError: cannot concatenate 'str' and 'int' objects" exception raised, but got following result.

>>> def g():
...     '1' + 0
...     yield 1, 2
...     yield 3, 4
...
>>> zip(*g())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: zip() argument after * must be a sequence, not generator
>>> (lambda xs: 0)(*g())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: <lambda>() argument after * must be a sequence, not generator
>>> list(*g())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: type object argument after * must be a sequence, not generator
>>> list(g())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in g
TypeError: cannot concatenate 'str' and 'int' objects

[ncoghlan]

It's not just hiding the real error, it's also saying something that isn't true. If you remove the deliberate error from the generator definition, it is clear that generators are perfectly acceptable as *arg inputs:

>>> def g(): yield 1, 2
... 
>>> list(*g())
[1, 2]

[falsetru]

Some exceptions are reported correctly.

>>> def g():
...     1 / 0
...     yield 1, 2
...     yield 3, 4
... 
>>> zip(*g())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in g
ZeroDivisionError: integer division or modulo by zero



>>> def g():
...     [][0]
...     yield 1, 2
...     yield 3, 4
... 
>>> zip(*g())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in g
IndexError: list index out of range

[durban]

This may be the same/similar as bpo-4806 (which has a patch).