Don't use mutable types as default arguments

This isn't safe given the way python implements default arguments.
Basically python store a reference to the instance it was passed, and
then if that argument is not provided it uses the default. That means
that two calls to the same function get the same instance, if one of
them mutates that instance every subsequent call that gets the default
will receive the mutated instance. The idiom to this in python is to use
None and replace the None,

def in(value: str, container: Optional[List[str]]) -> boolean:
   return src in (container or [])

if there is no chance of mutation it's less code to use or and take
advantage of None being falsy. If you may want to mutate the value
passed in you need a ternary (this example is stupid):

def add(value: str, container: Optional[List[str]]) -> None:
    container = container if container is not None else []
    container.append(value)

I've used or everywhere I'm sure that the value will not be mutated by
the function and erred toward caution by using ternaries for the rest.

1 files changed, 2 insertions, 1 deletions

diff --git a/mesonbuild/mesonmain.py b/mesonbuild/mesonmain.py
index e7005e9..f80dfdd 100644
--- a/mesonbuild/mesonmain.py
+++ b/mesonbuild/mesonmain.py
@@ -67,7 +67,8 @@ class CommandLineParser:
         self.add_command('unstable-coredata', munstable_coredata.add_arguments, munstable_coredata.run,
                          help=argparse.SUPPRESS)
 
-    def add_command(self, name, add_arguments_func, run_func, help, aliases=[]):
+    def add_command(self, name, add_arguments_func, run_func, help, aliases=None):
+        aliases = aliases or []
         # FIXME: Cannot have hidden subparser:
         # https://bugs.python.org/issue22848
         if help == argparse.SUPPRESS: