validate_car: Don't reassign input variable

This makes it easier to track what's happening in the function

tilde-train.py

@@ -198,40 +198,40 @@ def validate_car(car: list[str]):
 ## (2) pads it vertically or on the right side if it is too short or if
 ## not all lines are the same length and (3) removes bad characters.
 
-  car = "\n".join(car)
-  car = ''.join([i if ord(i) < 128 else ' ' for i in car])
-  car = car.split("\n")
+  car_str = "\n".join(car)
+  car_str = ''.join([i if ord(i) < 128 else ' ' for i in car_str])
+  car_list = car_str.split("\n")
 
   ## remove blank lines from top and bottom of car,
   ## so we can estimate its true size.
-  while car[0].strip() == "":
-    car.pop(0) ## clear top
-  while car[(len(car)-1)].strip() == "":
-    car.pop() ## clear bottom
+  while car_list[0].strip() == "":
+    car_list.pop(0) ## clear top
+  while car_list[(len(car_list)-1)].strip() == "":
+    car_list.pop() ## clear bottom
 
   ## len(choochoo_list) is the height of the train car, in number of rows.
-  if len(car) > max_y+1 or len(car) == 0:
+  if len(car_list) > max_y+1 or len(car_list) == 0:
     return 0  ## train car too tall or non-existant; skip it.
 
   ## vertically pad short cars with 1 space (lines will be lengthened later).
-  while len(car) < max_y:
-    car = [" "] + car
+  while len(car_list) < max_y:
+    car_list = [" "] + car_list
 
-  for idx,row in enumerate(car):
-    car[idx] = row.rstrip()
+  for idx,row in enumerate(car_list):
+    car_list[idx] = row.rstrip()
 
-  longest_line = len(max(car, key=len)) ## longest line in .choochoo file.
+  longest_line = len(max(car_list, key=len)) ## longest line in .choochoo file.
 
-  for idx,row in enumerate(car):
+  for idx,row in enumerate(car_list):
     if len(row) > max_x+1: ## check length of each row in .choochoo file.
       return 0 ## train car too long; skip it.
     elif "\t" in row or "\v" in row or "\f" in row or "\r" in row:
       return 0 # skip if contains tabs, vert tabs, line feeds or carriage ret
     elif len(row) < longest_line:
       padding = " "*(longest_line - len(row))
-      car[idx] += padding ## add padding spaces.
+      car_list[idx] += padding ## add padding spaces.
 
-  return car
+  return car_list
 
 
 def print_all_cars():