os.popen varies return type depending on exit status?

[David M. Wilson]

Hi there, on Linux:

>>> [ type(popen(cmd).close()) for cmd in ( 'true', 'foo' ) ]

sh: line 1: foo: command not found
[<type 'NoneType'>, <type 'int'>]


This feels inconsistent to me, and it just bit me because I was
expecting a 0 exit status on success, not None. Does close() return
this for historic reasons?


David.

 

[Donn Cave]

Hi there, on Linux:

[ type(popen(cmd).close()) for cmd in ( 'true', 'foo' ) ]

sh: line 1: foo: command not found
[<type 'NoneType'>, <type 'int'>]


This feels inconsistent to me, and it just bit me because I was
expecting a 0 exit status on success, not None. Does close() return
this for historic reasons?

The close function wraps either fclose(3) or pclose(3),
and I don't believe it actually knows which. They return
the same 0 value on success, so in this case you see fclose
behavior. Maybe that is for historic reasons, since in
principle I don't think it would hurt anything for fclose
to return 0.

Donn Cave, (e-mail address removed)