Remove some mutable default arguments.

These ones don't fix any bugs, because the mutable arg is never passed
outside of the callable or mutated.  But it's good practice to not use
them in case those invariants are changed in the future.

tools/lint-all

@@ -86,13 +86,13 @@ def check_pyflakes():
                 failed = True
     return failed
 
-def custom_check_file(fn, rules, skip_rules=[]):
+def custom_check_file(fn, rules, skip_rules=None):
     failed = False
     lineFlag = False
     for i, line in enumerate(open(fn)):
         skip = False
         lineFlag = True
-        for rule in skip_rules:
+        for rule in skip_rules or []:
             if re.match(rule, line):
                 skip = True
         if skip:
@@ -183,7 +183,7 @@ python_rules = [
      'exclude': set(['tools/lint-all']),
      'exclude_line': set([
          # function definition
-         ('zerver/lib/response.py', 'def json_error(msg, data={}, status=400):'),
+         ('zerver/lib/response.py', 'def json_error(msg, data=None, status=400):'),
          # No need to worry about the following as the translation strings
          # are already captured
          ('zerver/middleware.py',

zerver/lib/response.py

@@ -23,16 +23,17 @@ def json_method_not_allowed(methods):
         "allowed_methods": methods})
     return resp
 
-def json_response(res_type="success", msg="", data={}, status=200):
+def json_response(res_type="success", msg="", data=None, status=200):
     content = {"result": res_type, "msg": msg}
-    content.update(data)
+    if data is not None:
+        content.update(data)
     return HttpResponse(content=ujson.dumps(content) + "\n",
                         content_type='application/json', status=status)
 
-def json_success(data={}):
+def json_success(data=None):
     return json_response(data=data)
 
-def json_error(msg, data={}, status=400):
+def json_error(msg, data=None, status=400):
     return json_response(res_type="error", msg=msg, data=data, status=status)
 
 def json_unhandled_exception():