Question

I am trying to verify the validity of a string to ensure that it is a legal command I can pass to the terminal. If the string passes the test, I return True. Otherwise, I return False and an error message.

My code is pretty ugly with lots of nested if statements – how can I improve it?

task = task.split()
if len(task) > 1: 
    if task[0] == 'svn':
        if task[1] in ALLOWED:
            if len(task[2:]) == ALLOWED[task[1]]:
                return True, task, None
            else:
                return False, "Incorrect number of arguments."
        else:
            return False, "Not a legal command."    
    else:
        return False, "Not a subversion command."
else:
    return False, "Invalid input"

Answer 1

Instead of positive checks and nested if statements:

if a:
    if b:
        if c:
            foo()
        else:
            # error 3
     else:
         # error 2
else:
    # error 1

You can reverse the logic and bail out unless everything is OK:

if not a:
    # raise an exception

if not b:
    # raise an exception

if not c:
    # raise an exception

# If we get here, everything is OK.
foo()

This makes it easier to see which error message matches with which condition.