First bit of Python in a long time

Scuzz

My manager’s son is learning Python and for some reason he thinks i’m a pro in Python.
His son is doing some online course and he was struggling with a question.

def satisfiesF(L):
    """
    Assumes L is a list of strings
    Assume function f is already defined for you and it maps a string to a Boolean
    Mutates L such that it contains all of the strings, s, originally in L such
    that f(s) returns True, and no other elements. Remaining elements in L should be in the same order.
    Returns the length of L after mutation
    """
    # Your function implementation here

run_satisfiesF(L, satisfiesF)

After a while I managed to get something working and it passed 100% when he submitted it.

def f(s):
    return 'a' in s

def satisfiesF(L):
    
    for item in L:
        if not f(item)
            L.remove(item)
            
    return len(L)

L = ['a', 'b', 'a', 'josh', 'gary', 'mark', 'dave', 'chris']
print satisfiesF(L)
print L

Almost

Man, this is really awkward way to phrase a question about filtering.

Nice job, though. Good to see you doing shit again :P

EDIT: Is it safe to modify a list while iterating over it in python? A quick google search suggests no.

Scuzz

See, i kinda figured that out while i was testing some things.
It would print out incorrect data as the list was getting smaller, skipping some strings.

I then tried with a second empty list that i then made L equal to. This would not modify L globally which is what this question wanted.

EDIT:

Modified the code to be what i think is “correct”

def f(s):
    return 'b' in s

def satisfiesF(L):
    
    x = []
    for item in L:
        if f(item):
            x.append(item)  
    return x

L = ['a', 'b', 'a', 'a', 'b', 'a','a', 'b', 'a',]
L = satisfiesF(L)
print len(L)
print L

I think the example test code and the question are giving off the wrong idea as they want you to make global changes to L from within the function which can only be done via returning a value from the function. The test code says you need to return the length of L and make changes to L globally from within the function. You can use global to make global changes but doing this declares L as global and local which throws errors.

Seems a shitty way of asking a question and using shitty example code?

def f(s):
    return 'a' in s
      
L = ['a', 'b', 'a']
print satisfiesF(L)
print L

#Should print:
#2
#['a', 'a']

Almost

Maybe they expected you to do two passes?

def satisfiesF(L):
    x = []
    for item in L:
        if f(item):
            x.append(item)

    for item in x:
        L.remove(item)

    return len(L)

Seems like a bad question to me.

If we’re going with the second interpretation (where you return a new list) I think this would work:

def satisfiesF(L):
    return [x for x in L if f(x)]

First bit of Python in a long time

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