[issue15767] add ModuleNotFoundError

[Christoph Groth]

Christoph Groth  added the comment:

Unfortunately I do not feel competent enough to submit a documentation patch 
because I still do not understand why ModuleNotFoundError was added.

I don't want to bother you further with this.  Thank you all for your prompt 
replies.  If you agree with me that there is indeed an issue, please open it 
yourself.

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[issue15767] add ModuleNotFoundError

[Ned Deily]

Ned Deily  added the comment:

Christoph, thanks for your suggestion.  If you think the documentation needs 
improving, please open a new issue with any suggested wording (or, even better, 
a doc PR).  This issue (issue15767) has long been closed and any discussion 
here is likely to not be acted on.

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[issue15767] add ModuleNotFoundError

[Christoph Groth]

Christoph Groth  added the comment:

Thank you, Chris, for your reply.  If this is indeed the main use case of 
ModuleNotFoundError, I respectfully suggest to document it better.  The way 
things are now, Python users who switch to 3.6 encounter this new exception 
during their work with the interpreter and invariably wonder "Should I change 
anything in my code because of this?  If not, why was it introduced?".  In my 
opinion the current documentation does not answer these questions well.  Note 
that this is not about some deeply buried detail.  Every Python programmer is 
bound to encounter this.

That said, I cannot imagine many cases where user code would like to fall back 
to html.parser only if fancy_parser is not installed but not if an older 
version of fancy_parser is installed (or maybe it's an entirely *different* 
fancy_parser?).  And in the rare cases where that is desired, it was already 
perfectly idiomatic to do so:

try:
import fancy_parser
except ImportError:
from html.parser import HTMLParser
else:
from fancy_parser import NewParser as HTMLParser

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[issue15767] add ModuleNotFoundError

[Chris Jerdonek]

Chris Jerdonek  added the comment:

Eric touched on the use when he said the following above:

> It's nice to be able to distinguish between the failure to *find* the module 
> during import from other uses of ImportError.

To make up one example, you might want to use a fallback module if a package 
isn't installed:

try:
from fancy_parser import NewParser as HTMLParser
except ModuleNotFoundError:
from html.parser import HTMLParser

But you might still want an error if the package is installed, though 
incorrectly (e.g. fancy_parser is installed, but an old version that doesn't 
have NewParser). Catching ImportError would swallow this error, whereas 
ModuleNotFoundError would let it bubble up.

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[issue15767] add ModuleNotFoundError

[Christoph Groth]

Christoph Groth  added the comment:

In the above, please replace "understand the decision" by "understand the 
usefulness of it".

In the above discussion, as an alternative to a new exception, it was proposed 
to add an attribute to ImportError ('reason'), but then people seemed to agree 
that this is quite useless outside of importlib (msg192261).  But then I don't 
understand why the original idea of the exception was revived.

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[issue15767] add ModuleNotFoundError

[Christoph Groth]

Christoph Groth  added the comment:

My curiosity was piqued when I saw ModuleNotFoundError, so I decided to look it 
up.  This led me to this page and I read the complete discussion.  I still did 
not understand the decision, so I allowed myself to ask,  also because I 
believe that when new features are introduced it should be clear what they are 
good for.  That's all.

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum  added the comment:

I don't like the way you're asking questions here. If you're interested
just as a historian of the language, it  will have to wait. If you're
questioning the decision, please come out and say so.

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[issue15767] add ModuleNotFoundError

[Christoph Groth]

Christoph Groth  added the comment:

> Read Eric's message before mine.

Of course I read it, I wouldn't have asked otherwise.  Eric mentions an older 
message ("see msg182332") that *predates* your judgment that "outside importlib 
there shouldn't be a need to distinguish between the cases".  Otherwise Eric 
says that he finds "the exception to be very useful", but frankly, I don't see 
why (outside of importlib or backports of it).  What changed since msg192263?

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum  added the comment:

Read Eric's message before mine.

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[issue15767] add ModuleNotFoundError

[Christoph Groth]

Christoph Groth  added the comment:

I'm trying to understand why ModuleNotFoundError was added to 3.6.  The "what's 
new" entry links to this page.

Looking at the discussion, Guido said in 2013: "Right.  Outside importlib there 
shouldn't be a need to distinguish between the cases (especially given that the 
exception works differently than its name suggests).".  Then, three years 
later, he supports the inclusion of the patch, without that any new arguments 
had been given.

Could someone explain (or link to) what happened inbetween?

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[issue15767] add ModuleNotFoundError

[Eric Snow]

Changes by Eric Snow :


--
resolution:  -> fixed
stage: needs patch -> resolved
status: open -> closed

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[issue15767] add ModuleNotFoundError

[Roundup Robot]

Roundup Robot added the comment:

New changeset 90549c3c609c by Eric Snow in branch 'default':
Issue #15767: Add ModuleNotFoundError.
https://hg.python.org/cpython/rev/90549c3c609c

New changeset 5fdb8c897023 by Eric Snow in branch 'default':
Issue #15767: Use ModuleNotFoundError.
https://hg.python.org/cpython/rev/5fdb8c897023

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum added the comment:

+1 to add this to 3.6b1.

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[issue15767] add ModuleNotFoundError

[Eric Snow]

Changes by Eric Snow :


--
assignee: brett.cannon -> eric.snow
resolution: rejected -> 
stage:  -> needs patch
status: closed -> open
versions: +Python 3.6 -Python 3.4

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[issue15767] add ModuleNotFoundError

[Eric Snow]

Eric Snow added the comment:

Any chance we could revive ModuleNotFoundError?  It's nice to be able to 
distinguish between the failure to *find* the module during import from other 
uses of ImportError.  I'd definitely expect it to work the way Guido explained. 
 Basically only importlib._bootstrap._find_and_load_unlocked() would raise 
ModuleNotFoundError (when _find_spec() returns None).

I've found the exception to be very useful while working on the importlib 
backport (https://bitbucket.org/ericsnowcurrently/importlib2).  My desire for 
adding ModuleNotFoundError is unrelated to its internal use in importlib that 
motivated the original request (see msg182332).

Here's the signature:

  ModuleNotFoundError(*args, name=None), inherits from ImportError

For reference, here's ImportError:

  ImportError(*args, name=None, path=None)

ModuleNotFoundError would need to be exposed somewhere sensible since once 
people see in tracebacks they'll want to catch it. :)  I'd expect that to be 
either in builtins or as importlib.ModuleNotFoundError.  We may be able to get 
away with not adding it to builtins, but maybe it would still make sense.

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[issue15767] add ModuleNotFoundError

[Jesús Cea Avión]

Changes by Jesús Cea Avión j...@jcea.es:


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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


--
dependencies: +Update stdlib to use ModuleNotFoundError

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[issue15767] add ModuleNotFoundError

[Roundup Robot]

Roundup Robot added the comment:

New changeset 0e4e062751fa by Brett Cannon in branch 'default':
Issue #15767: Back out 8d28d44f3a9a related to ModuleNotFoundError.
http://hg.python.org/cpython/rev/0e4e062751fa

New changeset e3ec8b176a80 by Brett Cannon in branch 'default':
Issue #15767: Revert 3a50025f1900 for ModuleNotFoundError
http://hg.python.org/cpython/rev/e3ec8b176a80

New changeset ee9662d77ebb by Brett Cannon in branch 'default':
Issue #15767: back out 8a0ed9f63c6e, finishing the removal of
http://hg.python.org/cpython/rev/ee9662d77ebb

New changeset de947db308ba by Brett Cannon in branch 'default':
Issue #15767: Excise the remaining instances of ModuleNotFoundError
http://hg.python.org/cpython/rev/de947db308ba

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


--
resolution:  - rejected
status: open - closed

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[issue15767] add ModuleNotFoundError

[Eric Snow]

Eric Snow added the comment:

 I'm sorry, but this seems like it should be an importlib internal
 affair.  The new exception is too much in everyone's face, because
 the exception name gets printed on every traceback.

That's the crux of the issue.  If there isn't much utility outside importlib to 
distinguishing between module-not-found and other causes of ImportError, then 
there isn't much point to a new exception.  It just boils down to what the 
other potential causes of ImportError are and how much people care about them.

I keep thinking about PEP 3151 (IOError/OSError hierarchy rework) and the 
lessons we've learned about exception attributes vs. subclasses.  For 
readability and write-ability, I'd rather write this:

  try:
  from _collections import OrderedDict
  except ModuleNotFoundError:
  pass

than this:

  try:
  from _collections import OrderedDict
  except ImportError as e:
  if e.reason is not importlib.machinery.ImportReason.MODULE_NOT_FOUND:
  raise

But the relevant question is, what is the benefit (outside importlib) of either 
over this:

  try:
  from _collections import OrderedDict
  except ImportError:
  pass

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum added the comment:

Right.  Outside importlib there shouldn't be a need to distinguish between the 
cases (especially given that the exception works differently than its name 
suggests).

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[issue15767] add ModuleNotFoundError

[Thomas Heller]

Changes by Thomas Heller thel...@ctypes.org:


--
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[issue15767] add ModuleNotFoundError

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

If most user code should catch a ModuleNotFoundError exception, perhaps it will 
be better rename old ImportError to ImportlibError (or ImportingError, or 
ImportMachineryError, or BaseImportError) and new ModuleNotFoundError to 
ImportError. This will left most existing user code untouched.

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[issue15767] add ModuleNotFoundError

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

On Jul 02, 2013, at 07:16 AM, Serhiy Storchaka wrote:

If most user code should catch a ModuleNotFoundError exception, perhaps it
will be better rename old ImportError to ImportlibError (or ImportingError,
or ImportMachineryError, or BaseImportError) and new ModuleNotFoundError to
ImportError. This will left most existing user code untouched.

I urge some caution here.  I don't know that the above will cause problems,
but I do think you're walking into PEP territory.  I would feel much more
comfortable with an analysis based on testing existing third party code.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

OK, I'll revert the changes related to ModuleNotFoundError.

As for adding a 'reason' attribute, I see two sticking points. One is how to 
set the enum value. There is both the C code issue (specifically so ceval.c and 
import.c can use the values) as well as importlib's bootstrapping restrictions. 
I'll have to think about whether there is any reasonable way to make it work.

Second, as you hinted at Guido, is exactly what the acceptable cases may be. I 
would probably start with any ImportError raised directly by import itself and 
nothing more. Other things from loaders (e.g. bad magic number, stale bytecode, 
etc.) could be initially left out. That would mean the following possible 
values:

* module is not a package
* module not found
* None in sys.modules

But the bootstrapping issues for the enum module is probably going to be the 
showstopper for this. That suggests either adding not_found or figuring out 
some way to prevent _not_found from leaking outside of importlib (which I now 
think I can do somewhat reasonably).

--
resolution: fixed - 
stage: committed/rejected - 

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[issue15767] add ModuleNotFoundError

[Antoine Pitrou]

Antoine Pitrou added the comment:

 But the bootstrapping issues for the enum module is probably going
 to be the showstopper for this.

Have we succumbed to the enum religion already? Just make it a plain string.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

In this instance where there are only a set number of options are expected to 
be officially valid, yes I think enums are a good fit.

As for strings, the only way I would be okay with that is defining the strings 
either as attributes on ImportError itself or off of importlib to make it easy 
to do a comparison. But in that case I might as well just drop _not_found and 
use ``str(exc).startswith('No module named ')`` to detect what I need and be 
done with it.

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[issue15767] add ModuleNotFoundError

[Antoine Pitrou]

Antoine Pitrou added the comment:

 In this instance where there are only a set number of options are
 expected to be officially valid, yes I think enums are a good fit.

They are a good fit, that doesn't mean they're the only one.

 As for strings, the only way I would be okay with that is defining
 the strings either as attributes on ImportError itself or off of
 importlib to make it easy to do a comparison.

What does that mean?
I don't understand how `exc.reason == 'module_not_found'` is
harder than `exc.reason == ImportReason.MODULE_NOT_FOUND`.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

It's not a question of harder but more error-prone since a typo in the string 
won't be directly noticed while mistyping an attribute name will be.

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[issue15767] add ModuleNotFoundError

[Antoine Pitrou]

Antoine Pitrou added the comment:

 It's not a question of harder but more error-prone since a typo in
 the string won't be directly noticed while mistyping an attribute
 name will be.

Ok, agreed. I guess it's ok, if it only adds one or two attributes.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

Obviously no worries for opening this up again; if something isn't clear better 
to get it sorted out now rather than after 3.4 is out that door.

So I see two questions: why isn't ImportError being raised in the ``import 
re.bogus`` case and why the subtle difference in exception messages.

Let's deal with the latter first since it's the easiest: it's because that's 
how it was in Python 3.3::

 python3.3 
  ~
Python 3.3.2 (default, Jun 19 2013, 13:30:51) 
[GCC 4.2.1 Compatible Apple LLVM 4.2 (clang-425.0.28)] on darwin
Type help, copyright, credits or license for more information.
 import bogus
Traceback (most recent call last):
  File stdin, line 1, in module
ImportError: No module named 'bogus'

 from re import bogus
Traceback (most recent call last):
  File stdin, line 1, in module
ImportError: cannot import name bogus

 import re.bogus
Traceback (most recent call last):
  File frozen importlib._bootstrap, line 1521, in _find_and_load_unlocked
AttributeError: 'module' object has no attribute '__path__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File stdin, line 1, in module
ImportError: No module named 're.bogus'; re is not a package

All I did for ModuleNotFoundError is change what exception is raised. I didn't 
muck with any messages in adding this exception to keep the change minimal (at 
least to start with). It also doesn't help that in your case [2] (the ``from re 
import bogus`` case) is actually not handled by importlib but ceval.c. I really 
hate the semantics of ``from ... import`` and __import__ in this specific 
instance. Hopefully it can be something I can clean up in Python 4. Anyway, it 
can be updated to match: http://bugs.python.org/issue18342

As for the ``import re.bogus`` case not raising ModuleNotFoundError, I'm fine 
with changing it. I don't have a clear recollection as to why I chose to leave 
it as-is, but I also can't come up with a good reason to not change it.

And to explain why the ``from ... import`` case raises ModuleNotFoundError even 
when re is a module and obviously not a package is that while that might be 
true now, that does not necessarily hold in the future. If you have been using 
something like an object to hold attributes but then decide to switch to a 
module, this instance would break. Plus ``from ... import`` has never directly 
distinguished between a missing attribute and a missing module. Once again, 
something I would like to change in Python 4.

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum added the comment:

Ok, the history of all that makes sense, except now I wonder if 
ModuleNotFoundError is useful at all.  I just reviewed a patch for 3.4 and in 
the latest revision all ImportError checks were replaced with 
ModuleNotFoundError checks.  What purpose does that have except encouraging 
code churn?

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[issue15767] add ModuleNotFoundError

[Antoine Pitrou]

Antoine Pitrou added the comment:

Le lundi 01 juillet 2013 à 16:54 +, Brett Cannon a écrit :
 As for the ``import re.bogus`` case not raising ModuleNotFoundError,
 I'm fine with changing it.

+1.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

The impetus of ModuleNotFoundError is the need for something similar in 
importlib thanks to ``from ... import``. When you do something like 
__import__('os', fromlist=['path']), any ImportError must be silenced **if** it 
relates only to the fact that the module can't be found. Other errors directly 
related to technical errors which also raised ImportError are supposed to 
propagate.

Before ModuleNotFoundError I was setting a made-up attribute called _not_found 
on ImportError and then raising the exception. I could then catch the exception 
and introspect for that case as necessary. See 
http://hg.python.org/cpython/file/3e10025346c1/Lib/importlib/_bootstrap.py for 
the last revision that contained _not_found.

Since I had a legitimate case of wanting to differentiate between ImportError 
(which represents both technical errors and an inability to find a module) and 
the more specific case of simply not finding a module, I decided to formalize 
the distinction. It made sense to me so that the old try/except ImportError 
trick to ignore missing module could not accidentally catch the technical error 
use of ImportError by accident and mask the problem.

Now if you still think the subclass is not worth it then I probably need a 
better solution than a faked private attribute on ImportError to delineate the 
technical error/module not found difference as I had already received one bug 
report asking to define what _not_found was. ImportError could grow a not_found 
attribute formally or even a 'reason' attribute which could be set to some enum 
value representing the fact the exception was triggered because the module 
simply wasn't found (although that seems unnecessary and more like errno from 
OSError).

To summarize, the options I see are:

1. Back to _not_found on ImportError (only existed to start with as it was too 
late in 3.3 to come up with a better solution)

2. Keep ModuleNotFoundError (this option can include reverting all of the 
``except ImportError`` changes I made and just allow the exception to exist if 
you want)

3. Add a not_found attribute to ImportError more formally (defaults to None for 
undefined, set to True/False to clearly signal when an obvious answer is known)

4. Add a 'reason' attribute that can be set to an enum which has a value which 
represents module not found

I can't easily make it a private exception that I prevent from escaping 
importlib thanks to trying to allow the accelerated version of import to use 
importlib's fromlist code (see how _handle_fromlist is called with its import_ 
argument).

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum added the comment:

[switching to gmail-powered account]

I'm sorry, but this seems like it should be an importlib internal affair.  The 
new exception is too much in everyone's face, because the exception name gets 
printed on every traceback.

I like #4, #3 is also acceptable.  But #4 seems best because it can obviate a 
bunch of exception message parsing in user code, I'm sure.  Though we shouldn't 
go overboard with distinguishing cases, the two different places where you 
currently raise MNFE should be distinguished IMO.

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[issue15767] add ModuleNotFoundError

[Guido van Rossum]

Guido van Rossum added the comment:

Hey Brett,

Sorry for reopening this issue.  I am confused by the spec for 
ModuleNotFoundError.  Look at this (in a pretty recent repo):

$ ./python.exe
Python 3.4.0a0 (default:8f22e03f5f07, Jun 27 2013, 08:49:16) 
[GCC 4.2.1 Compatible Apple Clang 4.0 ((tags/Apple/clang-421.0.60))] on darwin
Type help, copyright, credits or license for more information.

[1]
 import bogus
Traceback (most recent call last):
  File stdin, line 1, in module
ModuleNotFoundError: No module named 'bogus'

[2]
 from re import bogus
Traceback (most recent call last):
  File stdin, line 1, in module
ModuleNotFoundError: cannot import name bogus

[3]
 import re.bogus
Traceback (most recent call last):
  File frozen importlib._bootstrap, line 1553, in _find_and_load_unlocked
AttributeError: 'module' object has no attribute '__path__'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File stdin, line 1, in module
ImportError: No module named 're.bogus'; re is not a package

Given that it knows that re is not a package, I would have expected [2] not to 
raise ModuleNotFoundError, because there is no way that bogus could be a 
package.  OTOH, I would have expected [3] to raise ModuleNotFoundError, since 
this syntax implies that bogus is a submodule.

But perhaps I am missing something and I need to look at the distinction 
differently?  Sadly the docs don't really help me; they claim to explain why I 
get ModuleNotFoundError in [2], but the motivation as the specific attribute 
being requested cannot be known a priori to be a module or some other type of 
object seems wrong, given that [3] proves it *does* know.

(Aside, it's also odd that bogus is quoted in the error message for [1] and [3] 
but not for [2] -- in fact the phrasing of [2] compared to [1] seems 
arbitrarily different, both seem to tell me exactly the same thing.)

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


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title: add ImportNotFoundError - add ModuleNotFoundError

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[issue15767] add ModuleNotFoundError

[Roundup Robot]

Roundup Robot added the comment:

New changeset 8a0ed9f63c6e by Brett Cannon in branch 'default':
Issue #15767: Introduce ModuleNotFoundError, a subclass of
http://hg.python.org/cpython/rev/8a0ed9f63c6e

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


--
resolution:  - fixed
stage: test needed - committed/rejected
status: open - closed

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[issue15767] add ModuleNotFoundError

[Roundup Robot]

Roundup Robot added the comment:

New changeset 3a50025f1900 by Brett Cannon in branch 'default':
Issue #15767: Touch up ModuleNotFoundError usage by import.
http://hg.python.org/cpython/rev/3a50025f1900

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[issue15767] add ModuleNotFoundError

[Roundup Robot]

Roundup Robot added the comment:

New changeset c6c0faaf65d7 by Brett Cannon in branch 'default':
Issue #15767: Add an explicit test for raising ModuleNotFoundError
http://hg.python.org/cpython/rev/c6c0faaf65d7

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[issue15767] add ModuleNotFoundError

[Antoine Pitrou]

Antoine Pitrou added the comment:

I've lost track of why this new exception was needed. Could someone provide a 
summary? Thanks :-)

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[issue15767] add ModuleNotFoundError

[Michele Orrù]

Changes by Michele Orrù maker...@gmail.com:


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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

TL;DR for Antoine: when using a fromlist, import failures from that list are 
silently ignored. Because ImportError is overloaded in terms of what it means 
(e.g. bag magic number, module not found) there needs to be a clean way to tell 
the import failed because the module wasn't found so other ImportError reasons 
can properly propagate.

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[issue15767] add ModuleNotFoundError

[Chris Jerdonek]

Chris Jerdonek added the comment:

 from foo import bar
 Here bar can be not module, but an attribute of foo (for example, os.path).
 Serhiy: What exception is raised in that situation is controlled by the eval 
 loop, not importlib so that would be a separate change.

Just to clarify from this exchange, is there a chance we will use this same 
exception type (perhaps in a later change) in cases where bar is not found?  If 
so, I think it's worth considering something like NotFoundImportError or 
ImportNotFoundError that doesn't single out module.  Importing classes, etc. 
is quite a common pattern (e.g. examples appear in PEP 8).

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[issue15767] add ModuleNotFoundError

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

from foo import bar

Here bar can be not module, but an attribute of foo (for example, os.path). 
What error will be raised in this case? Module or attribute - this is an 
implementation detail; why do we distinguish between these cases?

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

Eric: knock yourself out. =)

Serhiy: What exception is raised in that situation is controlled by the eval 
loop, not importlib so that would be a separate change. But regardless, there 
is no way to infer whether you expected an attribute or module to be there, 
just that you were after something that didn't exist. But I would argue most 
people import at the module level and not the attribute level, and so raising 
an ModuleNotFoundError would be acceptable.

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[issue15767] add ModuleNotFoundError

[Arfrever Frehtes Taifersar Arahesis]

Changes by Arfrever Frehtes Taifersar Arahesis arfrever@gmail.com:


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[issue15767] add ModuleNotFoundError

[Eric Snow]

Eric Snow added the comment:

+1 on just getting it done with ModuleNotFoundError.  FWIW, I'd be glad to do 
it.  I'm taking a self-imposed break from the nearly finished C-OrderedDict!

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[issue15767] add ModuleNotFoundError

[Barry A. Warsaw]

Changes by Barry A. Warsaw ba...@python.org:


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[issue15767] add ModuleNotFoundError

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

For me, it mostly comes down to whether end-users are expected to see such 
errors generally or not.  We see ImportErrors all the time, and they are 
clearly errors.  If we're expected to see and deal with MNF, and if in such 
cases it's generally considered an error, then MNFError is better.  If it's 
mostly an internal/hidden thing, then MNF doesn't bother me.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

Right, so what's typical? =) I mean do most people see ImportError for optional 
modules (e.g. not on support platforms), or do most people see ImportError 
because they messed up and tried to import something that they expected but 
actually isn't there for some reason.

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[issue15767] add ModuleNotFoundError

[Barry A. Warsaw]

Barry A. Warsaw added the comment:

On Feb 11, 2013, at 05:31 PM, Brett Cannon wrote:

Right, so what's typical? =) I mean do most people see ImportError for
optional modules (e.g. not on support platforms), or do most people see
ImportError because they messed up and tried to import something that they
expected but actually isn't there for some reason.

There are a few common use cases (or perhaps anti-use cases) where you see
ImportErrors.  I might be missing some, but I'd put these in roughly
descending order of commonness.

* Trying alternative imports for compatibility reasons.  You always expect
  ImportErrors in these cases, and you'll always catch them in try/excepts.

* Missing modules, submodules, or attributes in from-imports.  These can be
  unexpected if you think you've got the right version of a package, or
  expected for compatibility reasons.

* Trying to conditionally import optional modules.  Again, expected, and
  they'll be wrapped in try/except.

I guess the case you're trying to differentiate with MNF is, the from-import
case, i.e. did the error occur because the module was missing or because the
attribute was missing?

It's hard to say which is more likely, which I guess is why you're having a
hard time deciding. :) If I had to vote, I'd go with MNFError 1) because it's
a subclass of ImportError; 2) it'll be more informative in the case where it
really *is* an error; 3) isn't that big of a deal in cases where it's
expected; 4) we're used to seeing ImportError anyway, and probably most code
won't care and will just use ImportError.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

Screw it, I'll go with ModuleNotFoundError since it is a subclass of 
ImportError and so it might come off as weird as saying the superclass is an 
Error but the subclass is not.

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[issue15767] add ModuleNotFoundError

[Ezio Melotti]

Ezio Melotti added the comment:

I prefer ModuleNotFound.  Its meaning is already clear enough, even without the 
Error suffix.
OTOH we now have FileNotFoundError, but all the other OSError subclasses have 
that suffix.
In general I think the suffix is necessary when it's not already clear from the 
name that we are dealing with an exception, otherwise it can be dropped.

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[issue15767] add ModuleNotFoundError

[Chris Jerdonek]

Chris Jerdonek added the comment:

To state more explicitly the observation I alluded to in my comment above, we 
currently follow (without exception -- pun intended :) ) the convention that 
subclasses of exceptions that end in Error also end in Error.  We also do 
the same with the suffix Warning.

The latter is another reason to include the suffix Error -- to eliminate 
ambiguity as to whether the exception type inherits from a Warning class (e.g. 
from ImportWarning).

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[issue15767] add ModuleNotFoundError

[Ezio Melotti]

Ezio Melotti added the comment:

That seems indeed to be the case with built-in exceptions.  I'm not sure if 
it's intentional or just a coincidence.  I agree that warnings should always 
have a Warning suffix to distinguish them from exceptions, but in the stdlib 
the Error suffix is not used consistently. There are exceptions like: 
FloatOperation, DivisionByZero, InvalidOperation, TimeoutExpired, 
BrokenProcessPool, BufferTooShort, ImproperConnectionState, UnknownProtocol, 
InvalidURL, etc..
Anyway I don't have a strong opinion about this, so if you think the name 
should be ModuleNotFoundError it's OK with me (i.e. I'm -0).

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Brett Cannon added the comment:

Should it be ModuleNotFoundError or ModuleNotFound? It's not necessarily an 
error if a module doesn't exist, so failing to find it isn't quite that 
negative. It also acts somewhat like StopIteration/GeneratorExit which also 
don't have the common Error suffix of exception names.

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[issue15767] add ModuleNotFoundError

[Brett Cannon]

Changes by Brett Cannon br...@python.org:


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stage: needs patch - test needed

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[issue15767] add ModuleNotFoundError

[Chris Jerdonek]

Chris Jerdonek added the comment:

Since it subclasses ImportError, it seems like we're already considering it to 
be an error?  StopIteration and GeneratorExit don't inherit from an Error 
exception type unlike, say, the OSError exception types.

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[issue15767] add ModuleNotFoundError

[Chris Jerdonek]

Chris Jerdonek added the comment:

I meant to include this link for convenience:

http://docs.python.org/dev/library/exceptions.html#exception-hierarchy

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[issue15767] add ModuleNotFoundError

[Eric Snow]

Eric Snow added the comment:

I'd say ModuleNotFoundError.  You could argue that other exception types aren't 
really errors in certain circumstances.  I'd think that generally this would be 
an error.

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[issue15767] add ModuleNotFoundError

[Eric Snow]

Eric Snow added the comment:

Effectively the exception indicates that the import system had an error.

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[issue15767] add ModuleNotFoundError

[Andrew Svetlov]

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[issue15767] add ModuleNotFoundError

[Michele Orrù]

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[issue15767] add ModuleNotFoundError

[Chris Rebert]

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[issue15767] add ModuleNotFoundError

[Ezio Melotti]

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[issue15767] add ModuleNotFoundError

[Eric Snow]

New submission from Eric Snow:

In issue 15316 (msg168896, Brett Cannon):

  Create a ModuleNotFoundError exception that subclasses ImportError
  and catch that (breaks doctests and introduces a new exception that
  people will need to be aware of, plus the question of whether it
  should just exist in importlib or be a builtin)

While it's too late to go into 3.3, this is a reasonable addition for 3.4.  
Perhaps other ImportError subclasses are warranted, but they can be addressed 
separately.

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messages: 168906
nosy: brett.cannon, eric.snow
priority: normal
severity: normal
stage: needs patch
status: open
title: add ModuleNotFoundError
type: enhancement
versions: Python 3.4

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[issue15767] add ModuleNotFoundError

[Chris Jerdonek]

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