Author stestagg
Recipients Mark.Shannon, Mohamed_Atef, ethan.furman, gregory.p.smith, gvanrossum, josh.r, pablogsal, serhiy.storchaka, stestagg
Date 2021-01-13.10:55:35
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Marked as misclassified Yes
Message-id <1610535335.81.0.878058773539.issue42899@roundup.psfhosted.org>
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I re-read the change that introduced this, and the situation is slightly more complex.

While the wording is understandably slightly ambiguous, the change talks about the following example:

if a:
  pass
else:
  <do stuff>

In this scenario, the compiler is trying to eliminate the <pass> block, because it's redundant, and this seems totally fine to me.  The condition is still evaluated, but it's changing how the empty branch is handled.

Taking the example given in the ticket:
```
while True:
  if a:
     pass
  else:
     break
```

Before the change, the DIS is:

  2           0 NOP

  3     >>    2 LOAD_NAME                0 (a)
              4 POP_JUMP_IF_FALSE       10

  4           6 JUMP_FORWARD             0 (to 8)

  2     >>    8 JUMP_ABSOLUTE            2

  6     >>   10 LOAD_CONST               1 (None)
             12 RETURN_VALUE

  2          14 LOAD_CONST               1 (None)
             16 RETURN_VALUE

After the change, the DIS is:

  2           0 NOP

  3     >>    2 LOAD_NAME                0 (a)
              4 POP_JUMP_IF_FALSE       10

  4           6 NOP

  2           8 JUMP_ABSOLUTE            2

  6     >>   10 LOAD_CONST               1 (None)
             12 RETURN_VALUE

  2          14 LOAD_CONST               1 (None)
             16 RETURN_VALUE

Point being, the POP_JUMP_IF_FALSE is still invoked in the intended scenario, just some jumps are removed (OmG asserts this is faster).


So, If we test the simple case on MASTER, the following code still does the 'right thing'(tm) by evaluating bool(a):

```
class A:
    def __bool__(self):
        print("BOOL")
        return False

a = A()
if a:
   pass
```
prints "BOOL"

It seems like the issue is encountered when the "if" statement is mixed with (some other control flow scenarios, for example) exception handling:

```
class A:
    def __bool__(self):
        print("BOOL")
        return False

a = A()

try:
  if a:
     pass
except:
    raise
``
prints nothing


So, ignoring the larger discussion about if it's allowed to elide bool calls etc, this feels like an unintended side-effect of an otherwise non-contentious branch optimization, and we should either really understand the situation in-depth (all possible situations where this might happen), or just treat this as a bug and fix ti
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Date User Action Args
2021-01-13 10:55:35stestaggsetrecipients: + stestagg, gvanrossum, gregory.p.smith, ethan.furman, Mark.Shannon, serhiy.storchaka, josh.r, Mohamed_Atef, pablogsal
2021-01-13 10:55:35stestaggsetmessageid: <1610535335.81.0.878058773539.issue42899@roundup.psfhosted.org>
2021-01-13 10:55:35stestagglinkissue42899 messages
2021-01-13 10:55:35stestaggcreate