Pythonic way to set a maximum length parameter

What is the pythonic way to set a maximum length paramter?

Let's say I want to restrict a list of strings to a certain maximum size:

>>> x = ['foo', 'bar', 'a', 'rushmoreorless', 'kilimangogo']
>>> maxlen = 3
>>> [i for i in x if len(i) <= maxlen]
['foo', 'bar', 'a']

And I want to functionalize it and allow different maxlen but if no maxlen is given, it should return the full list:

def func1(alist, maxlen):
  return [i for i in x if len(i) <= maxlen]

And I want to set the maxlen to the max length of element in alist, so I tried but I am torn between using None, 0 or -1.

If I use None, it would be hard to cast the type later on:

def func1(alist, maxlen=None):
  if maxlen == None:
    maxlen = max(alist, key=len)
  return [i for i in x if len(i) <= maxlen]

And I might get this as the function builds on:

>>> maxlen = None
>>> int(None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: int() argument must be a string or a number, not 'NoneType'
>>> type(maxlen)
<type 'NoneType'>

If I use -1, it sort of resolve the int(maxlen) issues but it's also sort of weird since length should never be negative.

def func1(alist, maxlen=-1):
  if maxlen < 0:
    maxlen = max(alist, key=len)
  return [i for i in x if len(i) <= maxlen]

If I use 0, I face the problem when I really need to return an empty list and set maxlen=0 but then again, if I really need an empty list then I don't even need a function to create one.

def func1(alist, maxlen=0):
  if maxlen == 0:
    maxlen = max(alist, key=len)
  return [i for i in x if len(i) <= maxlen]

(Note: the ultimate task is NOT to filter a list, i.e. (filter(lambda k: len(k) <= maxlen, lst)) but it's an example to ask about the pythonic way to set an integer variable that sets a maximum)

Solution

Let's say I want to restrict a list of strings to a certain maximum size:

And I want to functionalize it and allow different maxlen but if no maxlen is given, it should return the full list:

And I want to set the maxlen to the max length of element in alist

To address all these requests, the best answer I can think of is something like this...

def func(lst, ln=None):
    if not ln:
        return lst
    ln = max(ln, len(lst))
    return [i for i in lst if len(i) <= ln]

edit:

If it is important to handle negative maximum length (who knows why) or 0 length, then a function like this can be used. (Though I am against duck-typing)

def func(lst, ln=None):
    if ln is None:
        return lst
    elif ln < 1:  # handles 0, and negative values.
        return []
    else:
        ln = max(ln, len(lst))
        return [i for i in lst if len(i) <= ln]