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.

2 files changed, 5 insertions, 4 deletions

diff --git a/mesonbuild/ast/interpreter.py b/mesonbuild/ast/interpreter.py
index a82ba5d..a75c0b7 100644
--- a/mesonbuild/ast/interpreter.py
+++ b/mesonbuild/ast/interpreter.py
@@ -22,7 +22,7 @@ from .. import environment
 from ..interpreterbase import InvalidArguments, BreakRequest, ContinueRequest
 
 import os, sys
-from typing import List
+from typing import List, Optional
 
 class DontCareObject(interpreterbase.InterpreterObject):
     pass
@@ -46,9 +46,9 @@ ADD_SOURCE = 0
 REMOVE_SOURCE = 1
 
 class AstInterpreter(interpreterbase.InterpreterBase):
-    def __init__(self, source_root: str, subdir: str, visitors: List[AstVisitor] = []):
+    def __init__(self, source_root: str, subdir: str, visitors: Optional[List[AstVisitor]] = None):
         super().__init__(source_root, subdir)
-        self.visitors = visitors
+        self.visitors = visitors if visitors is not None else []
         self.visited_subdirs = {}
         self.assignments = {}
         self.assign_vals = {}
diff --git a/mesonbuild/ast/introspection.py b/mesonbuild/ast/introspection.py
index 5a12e29..f8fb2e8 100644
--- a/mesonbuild/ast/introspection.py
+++ b/mesonbuild/ast/introspection.py
@@ -35,7 +35,8 @@ class IntrospectionHelper:
 class IntrospectionInterpreter(AstInterpreter):
     # Interpreter to detect the options without a build directory
     # Most of the code is stolen from interperter.Interpreter
-    def __init__(self, source_root, subdir, backend, visitors=[], cross_file=None, subproject='', subproject_dir='subprojects', env=None):
+    def __init__(self, source_root, subdir, backend, visitors=None, cross_file=None, subproject='', subproject_dir='subprojects', env=None):
+        visitors = visitors if visitors is not None else []
         super().__init__(source_root, subdir, visitors=visitors)
 
         options = IntrospectionHelper(cross_file)