int([x[, base]]). Square brackets in functions in Python documentation?

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.