[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 2b5cbbb13c6c9138d04c3ca4eb7431f8c65d8e65 by Serhiy Storchaka in 
branch '3.6':
[3.6] bpo-21720: Restore the Python 2.7 logic in handling a fromlist. (GH-4118) 
(#4128)
https://github.com/python/cpython/commit/2b5cbbb13c6c9138d04c3ca4eb7431f8c65d8e65


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan  added the comment:

Given that the automated cherry-pick failed, I'd consider a 3.6 backport nice 
to have, but definitely not essential.

My rationale for that is that "from __future__ import unicode_literals" makes 
it fairly easy to stumble over the 2.7 variant of this error message, but we're 
not aware of a similarly implicit way of encountering the 3.x variant.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Is it worth to backport PR 4118 to 3.6?

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:


New changeset 41c56940c6edf3ea169332a6b039b6c8796f0475 by Serhiy Storchaka in 
branch 'master':
bpo-21720: Restore the Python 2.7 logic in handling a fromlist. (#4118)
https://github.com/python/cpython/commit/41c56940c6edf3ea169332a6b039b6c8796f0475


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan  added the comment:

Serhiy's PR avoids the cryptic BytesWarning on Py3 while minimising the 
overhead of the new typecheck, so I've closed Berker's PR in favour of that one 
(which now has approved reviews from both Brett and I, so Serhiy will merge it 
when he's ready to do so).

Thanks for both patches!

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Tal Einat]

Tal Einat  added the comment:

I understand that there is a workaround. I'm just thinking about the many 
existing large codebases where re-writing thousands of imports because of this 
is unlikely to be done, yet having somewhat longer process launch times would 
be surprising and unwanted.

Anyways I do think it's a very small price to pay for better error messages, 
and there's a good chance nobody will actually feel the difference, so let's 
definitely move forward with this.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Brett Cannon]

Brett Cannon  added the comment:

As Nick said, if the overhead of an import statement is that critical, then you 
should NOT use the `from ... import ...` form at all and just stick with 
`import ...` and if necessary, bind local names to objects off of the final 
module or a local name for the overall module.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Tal Einat]

Tal Einat  added the comment:

I can't say I agree that the performance here is practically insignificant. 
This will affect the startup time of Python process, and adding even 10% to 
that in some cases is significant.

In some of the larger codebases I've worked on, even simple scripts would 
import large portions of the system, and there would be thousands of such 
imports done in the process. There are "basic" utilities in the stdlib which 
are imported very often in different modules, so the performance of the import 
statement is not necessarily insignificant compared to that of actually loading 
the modules.

That being said I'm all for getting this in and implementing an optimization of 
the slower path in time for 3.7.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

There are other differences between Python 2.7 and Python 3. PR 4118 restores 
the Python 2.7 logic. It adds type checking, but its overhead is smaller.

$ ./python -m timeit 'from encodings import aliases'
50 loops, best of 5: 542 nsec per loop

Actually in some cases (with '*') the new code is even slightly faster.

I don't know whether these differences were intentional, but all tests are 
passed.

The type of items in __all__ also is checked.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

The from import already is much slower than simple import:

$ ./python -m timeit 'from encodings import aliases'
50 loops, best of 5: 475 nsec per loop
$ ./python -m timeit 'import encodings.aliases as aliases'
100 loops, best of 5: 289 nsec per loop

The latter executes only C code if the module already is imported, but the 
former executes Python code. It may be worth to add the C acceleration for this 
case too.

PR 4113 makes it yet slower:
 
$ ./python -m timeit 'from encodings import aliases'
50 loops, best of 5: 793 nsec per loop

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan  added the comment:

I'm fine with the approach in the latest version of the PR - it does make "from 
x import y" slightly slower due to the extra error checking, but folks should 
be avoiding doing that in performance critical loops or functions anyway.

It would be nice if we could avoid that overhead for the import statement case, 
but we can't readily tell the difference between "__import__ called via syntax" 
and "__import__ called directly", and I don't think this is going to be 
performance critical enough to be worth introducing that complexity.

The question of how best to handle passing a consumable iterator as the from 
list would be a separate question, if we decided to do anything about it at all 
(the current implementation implicitly assumes the input is reiterable, but 
passing a non-container seems like a harder mistake to make than accidentally 
passing bytes instead of a string).

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag  added the comment:

> I don't think the index in error message is needed.

I'm fine with either format. It's ultimately up to Nick. Should I switch back 
to the 2.7 version?

> Import is successful because the iterator was exhausted by "'*' in fromlist".

This shouldn't be a problem in the latest version of PR 4113. While I don't 
think anyone would pass ``fromlist=iter(('aliases', b'foobar'))`` in real life, 
I can add it to the test. Let me know what do you think.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

>>> __import__('encodings', fromlist=iter(('aliases', b'foobar')))


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

I don't think the index in error message is needed. Unlike to str.join() which 
accepts arbitrary iterables of arbitrary names, the fromlist usually is a short 
tuple.

Interesting, what happen if the fromlist is not a list or tuple?

>>> __import__('encodings', fromlist=iter(('aliases', b'codecs')))


Import is successful because the iterator was exhausted by "'*' in fromlist".

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Change by Berker Peksag :


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag  added the comment:

issue21720_python3.diff hasn't been applied. I'll convert my 
issue21720_python3.diff patch to a pull request. I like the format of Nick's 
"".join() example in msg278794. Here's my proposal for Python 3:

f"Item {i} in 'from list' must be str, not {type(x).__name__!r}"

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Change by Serhiy Storchaka :


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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Ben Finney]

Ben Finney  added the comment:

On 23-Oct-2017, Serhiy Storchaka wrote:
> Is it all with this issue?

I accept Nick's reasoning:

> As a general principle, we don't give value information in type
> errors, since the error is structural rather than value based.

as sufficient to reject my requested improvement to the message.

The change in  is
good enough, IMO.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka  added the comment:

Is it all with this issue?

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan added the comment:

@Berker: the warning under "-bb" is a separate issue related to the handling of 
wildcard imports (_handle_fromlist searches for '*' and then pops it from a 
copy of the input list, replacing it with __all__ if that's defined)

@Ben: As a general principle, we don't give value information in type errors, 
since the error is structural rather than value based. The closest equivalent 
to us doing that that I'm aware of is the sequence index being given in 
str.join's TypeError:

>>> "".join(["a", "b", b"c"])
Traceback (most recent call last):
  File "", line 1, in 
TypeError: sequence item 2: expected str instance, bytes found

By contrast, when sorting, we don't give *any* indication as to where in the 
sequence the problem was found or the specific values involved:

>>> sorted(["a", "b", b"c"])
Traceback (most recent call last):
  File "", line 1, in 
TypeError: unorderable types: bytes() < str()

That doesn't make it a bad idea (as I think you're right that it would often 
make debugging easier), I'd just prefer to consider that as a separate question 
rather than introducing a one-off inconsistency with the general policy here 
(in particular, encountering TypeError is far more likely with str.join and 
sorted than it is with __import__, so it would be genuinely weird for the 
latter to have the most helpful error message).

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Ben Finney]

Ben Finney added the comment:

On 16-Oct-2016, Roundup Robot wrote:

> New changeset 7dd0910e8fbf by Berker Peksag in branch '2.7':
> Issue #21720: Improve exception message when the type of fromlist is unicode
> https://hg.python.org/cpython/rev/7dd0910e8fbf

This is an improvement, but it still should IMO show *which* item
caused the error.

Can it state “Item in from list must be str, not {type}: {item!r}”?
That would show the exact item so the reader has a better chance at
finding where it came from.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag added the comment:

Thanks for the reviews! I pushed the patch for 2.7.

Nick, what do you think about the case Serhiy mentioned in msg278515? Should we 
fix it or is issue21720_python3.diff good to go?

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Roundup Robot]

Roundup Robot added the comment:

New changeset 7dd0910e8fbf by Berker Peksag in branch '2.7':
Issue #21720: Improve exception message when the type of fromlist is unicode
https://hg.python.org/cpython/rev/7dd0910e8fbf

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Ben Finney]

Ben Finney added the comment:

On 12-Oct-2016, Nick Coghlan wrote:

> In this case, I think just reporting the first failing item is fine,
> and mentioning the type of that item in the error message is the
> most useful additional information we can provide to make things
> easier to debug.

Yes; also, the type expected, so the user knows what's different from
expected.

That is, the error message should say exactly what type is expected
*and* exactly what type failed that check.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan added the comment:

Right, Python 2.7 import is what it is at this point - if folks want something 
more modern, they can take a look at importlib2 :)

In this case, I think just reporting the first failing item is fine, and 
mentioning the type of that item in the error message is the most useful 
additional information we can provide to make things easier to debug.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag added the comment:

Well, I find using a for loop is a bit verbose in this case :)

In Python 3.2:

>>> __import__('encodings', fromlist=[b'aliases'])
Traceback (most recent call last):
  File "", line 1, in 
TypeError: Item in ``from list'' not a string

BytesWarning is there since Python 3.3 and I couldn't find any report on the 
tracker. I'd be fine with either committing the current patch or using 
pre-importlib exception message from 3.2 in 3.5+ (assuming the chance of 
passing a non-str item is low in Python 3)

> Other solution for Python 2 is allowing unicode in fromlist.

My preference is to improve the exception message and move on. Accepting both 
str and unicode would make the code harder to maintain and it would be better 
to avoid potential regressions in 2.7 (note that we already introduced 
regressions in the previous bugfix releases :))

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

> How about raising a TypeError if ``all(isinstance(l, str) for l in
> fromlist)`` is False? That would make the exception message less clearer
> since we can't include the "[...] not 'bytes'" part though.

for l in fromlist:
if not isinstance(l, str):
raise TypeError(...)

Note also that you can get an arbitrary error if fromlist is not a sequence.

Actually this issue doesn't look very important for Python 3, since it is 
unlikely that non-string is passed in fromlist. Unlike to Python 2 where you 
can pass unicode if just use unicode_literals. Other solution for Python 2 is 
allowing unicode in fromlist. Perhaps this would increase compatibility with 
Python 3.

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Item in ``from list'' not a string" message

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag added the comment:

> Should we file a separate issue regarding the similarly vague error message 
> from hasattr() itself?

+1 from me. It would be good to show users a user friendly message :)

> BytesWarning: Comparison between bytes and string

How about raising a TypeError if ``all(isinstance(l, str) for l in fromlist)`` 
is False? That would make the exception message less clearer since we can't 
include the "[...] not 'bytes'" part though.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

If run Python 3 with -bb:

>>> __import__('encodings', fromlist=[b'aliases'])
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 1000, in _handle_fromlist
BytesWarning: Comparison between bytes and string

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan added the comment:

Thanks Berker - looks good to me!

Should we file a separate issue regarding the similarly vague error message 
from hasattr() itself?

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag added the comment:

Good catch, thanks Nick. Here's a Python 3 version of the patch. I excluded 
Python/importlib.h from the patch to make review easier.

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Added file: http://bugs.python.org/file45063/issue21720_python3.diff

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Nick Coghlan]

Nick Coghlan added the comment:

Berker's fix for Python 2.7 looks good to me.

However, Python 3 has a comparably vague error message, it's just inverted to 
complain about bytes rather than unicode due to the change in the native str 
type:

>>> __import__('encodings', fromlist=[b'aliases'])
Traceback (most recent call last):
  File "", line 1, in 
  File "", line 1013, in _handle_fromlist
TypeError: hasattr(): attribute name must be string

hasattr() in Python 2.7 is similarly unhelpful regarding what type it actually 
got when you give it something it doesn't expect.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag added the comment:

Here's a patch to demonstrate what I meant in msg226047.

Example from the REPL:

>>> __import__('encodings', fromlist=[u'aliases'])
Traceback (most recent call last):
  File "", line 1, in 
TypeError: Item in ``from list'' must be str, not unicode

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Berker Peksag]

Berker Peksag added the comment:

I think we can classify this one as a usability bug and improve the exception 
message.

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[issue21720] "TypeError: Item in ``from list'' not a string" message

[Tim Graham]

Tim Graham added the comment:

As far as I can tell, this isn't an issue on Python 3. Can this be closed since 
Python 2 is only receiving bug fixes now?

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[issue21720] TypeError: Item in ``from list'' not a string message

[Ben Finney]

Ben Finney added the comment:

Is there room for a better resolution: fix the API so that Unicode objects are 
accepted in the ‘fromlist’ items?

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[issue21720] TypeError: Item in ``from list'' not a string message

[Ben Finney]

Changes by Ben Finney ben+pyt...@benfinney.id.au:


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[issue21720] TypeError: Item in ``from list'' not a string message

[David Szotten]

David Szotten added the comment:

not sure i follow. we need a different message if e.g. an integer is passed in

updated the patch to only run the unicode check for non-strings

or do you have a suggestion for an error message that works nicely in both 
cases?

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[issue21720] TypeError: Item in ``from list'' not a string message

[Julian Gindi]

Julian Gindi added the comment:

I'm trying to replicate this issue. I do not get an error when I run

```
  __import__('datetime', fromlist=[u'datetime'])
```

any more information you could provide to help move this issue forward?

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[issue21720] TypeError: Item in ``from list'' not a string message

[David Szotten]

David Szotten added the comment:

after some trial and error it only appears to break for 3rd party packages (all 
20 or so i happened to have installed), whereas everything i tried importing 
from the standard library worked fine

```
 __import__('requests.', fromlist=[u'get'])
Traceback (most recent call last):
  File stdin, line 1, in module
TypeError: Item in ``from list'' not a string

 __import__('os.', fromlist=[u'path'])
module 'os' from 'snip (virtualenv)/lib/python2.7/os.pyc'
```

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[issue21720] TypeError: Item in ``from list'' not a string message

[Julian Gindi]

Julian Gindi added the comment:

Interesting...I'll try to dig in and see what's going on.

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[issue21720] TypeError: Item in ``from list'' not a string message

[David Szotten]

David Szotten added the comment:

first ever patch to python, so advice on the patch would be appreciated

found an example in the stdlib that triggers bug (used in test):

`__import__('encodings', fromlist=[u'aliases'])`

--
keywords: +patch
Added file: http://bugs.python.org/file36500/fromlist.patch

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[issue21720] TypeError: Item in ``from list'' not a string message

[Berker Peksag]

Berker Peksag added the comment:

Thanks for the patch, David!

+def test_fromlist_error_messages(self):
+# Test for issue #21720: fromlist unicode error messages
+try:
+__import__('encodings', fromlist=[u'aliases'])
+except TypeError as exc:
+self.assertIn(must be str, not unicode, str(exc))

You could use assertRaises here:

with self.assertRaises(TypeError) as cm:
# ...

self.assertIn('foo', str(cm.exception))


+if (PyUnicode_Check(item)) {
+PyErr_SetString(PyExc_TypeError,
+Item in ``from list'' must be str, not unicode);
+Py_DECREF(item);
+return 0;
+}

I think it would be better to improve the error message in Python/import.c:

http://hg.python.org/cpython/file/2.7/Python/import.c#l2571

So you can safely remove this check.

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stage: needs patch - patch review

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[issue21720] TypeError: Item in ``from list'' not a string message

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


--
assignee:  - rhettinger
nosy: +rhettinger

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[issue21720] TypeError: Item in ``from list'' not a string message

[Raymond Hettinger]

Changes by Raymond Hettinger raymond.hettin...@gmail.com:


--
assignee: rhettinger - 

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[issue21720] TypeError: Item in ``from list'' not a string message

[Berker Peksag]

Changes by Berker Peksag berker.pek...@gmail.com:


--
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[issue21720] TypeError: Item in ``from list'' not a string message

[Ezio Melotti]

Ezio Melotti added the comment:

Do you want to propose a patch?
I think the standard message in these cases is along the lines of TypeError: 
fromlist argument X must be str, not unicode

--
keywords: +easy
nosy: +ezio.melotti
stage:  - needs patch

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[issue21720] TypeError: Item in ``from list'' not a string message

[David Szotten]

New submission from David Szotten:

```
 __import__('fabric.', fromlist=[u'api'])
Traceback (most recent call last):
  File stdin, line 1, in module
```

accidentally ended up with something like this via some module that was using 
`unicode_literals`. stumped me for a second until i realised that my variable 
was a string, but not `str`. would be nice with a custom error message if this 
is a unicode string, explicitly mentioning that these must not be unicode or 
similar

--
messages: 220267
nosy: davidszotten
priority: normal
severity: normal
status: open
title: TypeError: Item in ``from list'' not a string  message
type: enhancement
versions: Python 2.7

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[issue21720] TypeError: Item in ``from list'' not a string message

[Berker Peksag]

Changes by Berker Peksag berker.pek...@gmail.com:


--
nosy: +brett.cannon, eric.snow

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