方括号内带有`or`的python语句

Question

This is a code snippet I pasted from a setup.py python file. I am new to python and don't understand this build_args variable. Could someone give me some explanation for that?

build_args = [NINJA or MAKE]

# control the number of concurrent jobs
if self.jobs is not None:
    build_args.extend(['-j', str(self.jobs)])

subprocess.check_call(build_args)

Answer 1

build_args' instantiation is simply evaluating a logical boolean OR statement inside of the list a list structure. After the OR statement has been evaluated there will simply be a single boolean value stored in build_args. (props to ukemi, beat me to the punch)

Since it was also included in the code snippet I'd add that info for '.extend()' following can be found here . Essentially .extend() then just appends all items of an iterable structure on the end of the list, so build_args' content would then be [<boolean>, '-j', <job_string>]

Answer 2

Use:

build_args = [NINJA or MAKE]

• If NINJA is "truthy", then build_args = [NINJA]
• If NINJA is "falsy", then build_args = [MAKE]

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Note: Python's Truthy and Falsy - generalized Booleans

In python there are other values equivalent to True and False , other than the booleans themselves:

In the context of Boolean operations, and also when expressions are used by control flow statements, the following values are interpreted as false:

• False
• None
• numeric zero of all types
• empty strings and containers (including strings, tuples, lists, dictionaries, sets and frozensets)

All other values are interpreted as true.

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Truth table for or :

x	y	x or y
True	True	x
True	False	x
False	True	y
False	False	y