问题
What is the meaning of the square brackets inside the round brackets of a function in the Python documentation?
E.g.:
help([object])
or
int([x[, base]])
回答1:
Everything that is in square brackets is optional, i.e. you can omit it. If the square brackets contain more than 1 argument, you can't choose which ones to omit, you must either specify all of them, or none.
That's where nested brackets come in handy:
int([x[, base]])
Here, for example, you can use int() without arguments (by omitting the whole outer bracket) or int(x) (by omitting the inner bracket) or int(x, base). But not int(base) (well, that would just mean int(x)).
This isn't actual Python syntax, just a way for documentation to be clearer. Python 3's documentation tries to avoid these brackets.
回答2:
These are optional arguments. You need not specify them, but you may want to use them for specific functionality.
When one or more top-level parameters have the form parameter = expression, the function is said to have “default parameter values.” For a parameter with a default value, the corresponding argument may be omitted from a call, in which case the parameter’s default value is substituted. If a parameter has a default value, all following parameters must also have a default value — this is a syntactic restriction that is not expressed by the grammar.
source
来源：https://stackoverflow.com/questions/10053286/intx-base-square-brackets-in-functions-in-python-documentation