[Python-ideas] Potential PEP: with/except

[Chris Angelico]

On Wed, Jan 23, 2019 at 9:23 AM Paul Ferrell <pflarr at gmail.com> wrote:
>
> That is definitely an ambiguity worth considering (whether __enter__ is within the implied 'try').
>

It's not even __enter__.

>>> import dis
>>> def f(path):
...  with open(path) as f:
...   print("Got f:", f)
...
>>> dis.dis(f)
  2           0 LOAD_GLOBAL              0 (open)
              2 LOAD_FAST                0 (path)
              4 CALL_FUNCTION            1
              6 SETUP_WITH              16 (to 24)
              8 STORE_FAST               1 (f)

  3          10 LOAD_GLOBAL              1 (print)
             12 LOAD_CONST               1 ('Got f:')
             14 LOAD_FAST                1 (f)
             16 CALL_FUNCTION            2
             18 POP_TOP
             20 POP_BLOCK
             22 BEGIN_FINALLY
        >>   24 WITH_CLEANUP_START
             26 WITH_CLEANUP_FINISH
             28 END_FINALLY
             30 LOAD_CONST               0 (None)
             32 RETURN_VALUE
>>>

At the time when "open(path)" is called, the 'with' block hasn't begun
operating yet. The SETUP_WITH operation will call __enter__, but prior
to that, we have to have an object to use as the context manager. Any
exception thrown by opening the file will happen before __enter__ gets
called. AIUI the __enter__ method of file objects doesn't actually do
anything much (just validates that it's still open).

ChrisA