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Drop superfluous 'global' statements from command.py #621

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May 25, 2024
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Drop superfluous 'global' statements from command.py #621

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Drop superfluous 'global' statements from command.py
cottsay Feb 22, 2024
c4ee7d4
Merge branch 'master' into cottsay/superfluous-global-statements
cottsay May 24, 2024
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4 changes: 0 additions & 4 deletions colcon_core/command.py
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Original file line number Diff line number Diff line change
Expand Up @@ -86,7 +86,6 @@ def register_command_exit_handler(handler):

:param handler: The callable
"""
global _command_exit_handlers
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I don't think is trivial removing global in relation to _command_exit_handlers that variable is defined as a global variable in line

colcon-core/colcon_core/command.py

Line 78 in 03d6193

_command_exit_handlers = []
.
The prefix global in python is use to be able to read and write a global variable in the context of a specific function. Without that prefix whatever change you do to that variable within the function won't take effect on the global defined variable but a copy of it.

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Without that prefix whatever change you do to that variable within the function won't take effect on the global defined variable but a copy of it.

I don't think this statement is true as written. The global statement makes value replacement operations affect the global variable instead of creating a local variable, but MODIFYING an object behind a global variable is not affected by the global statement at all, and has always been the implicit behavior.

>>> foo = []
>>> def doit():
...   foo.append('hello, world')
... 
>>> doit()
>>> foo
['hello, world']
>>>

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With a replacement-type operation, the global statement is needed:

>>> foo = 0
>>> def doit():
...   foo = 1
... 
>>> doit()
>>> foo
0
>>> def doit_better():
...   global foo
...   foo = 1
... 
>>> doit_better()
>>> foo
1
>>>

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It seems I have been coding immutable code for long enough that I have forgotten that a lot of data structures are mutable. For the use case of this code, you are right and as long as we are not doing value replacements, there is no explicit need of the global prefix.
I would even argue in favor of not having the global prefix, since it removes the effect of replacing the value of the global variable and could prevent some unexpected behavior (aka someone adds a line like foo =['this won't do anything']).

if handler not in _command_exit_handlers:
_command_exit_handlers.append(handler)
Comment on lines 89 to 90
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I'm curious if this runs successfully in terms of finding the correct reference to the global variable. From my understanding it should fail since _command_exit_handlers is not defined in the function. (But there might be some python juju magic in place to make it kinda work)


Expand All @@ -113,7 +112,6 @@ def main(*, command_name='colcon', argv=None):
:param list argv: The list of arguments
:returns: The return code
"""
global _command_exit_handlers
try:
return _main(command_name=command_name, argv=argv)
except KeyboardInterrupt:
Expand All @@ -126,7 +124,6 @@ def main(*, command_name='colcon', argv=None):


def _main(*, command_name, argv):
global colcon_logger
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I think in terms of the logger it's possible to remove the global prefix since we never modify the value of it directly but through methods of the same logger.

# default log level, for searchability: COLCON_LOG_LEVEL
colcon_logger.setLevel(logging.WARNING)
set_logger_level_from_env(
Expand Down Expand Up @@ -375,7 +372,6 @@ def create_subparser(parser, cmd_name, verb_extensions, *, attribute):
selected verb
:returns: The special action object
"""
global colcon_logger
assert verb_extensions, 'No verb extensions'

# list of available verbs with their descriptions
Expand Down
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