How do I find where the first call of the function was and return None accordingly?

user_13622790 0
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I was given the following question:

Write a function last_in(x) that returns the value of x used in the previous call. The first time that the function is called, it will return None.

I did something that somewhat works, but something is missing. Here's my attempt:

def last_in(x):
    def inner():
        return x

    return inner()

And the output of

print(last_in(3),last_in(4),last_in(5), last_in("a"),last_in("12"))

is

3 4 5 a 12

But the expected needs to be

None 3 4 5 a

What do I miss?

Thank you!

python-3.x closures python
6 Answers

You are creating a function inner and immediately calling it. So this:

def last_in(x):
    def inner():
        return x

    return inner()

is roughly equivalent to this:

def last_in(x):
    return x

What you can do is, define a variable and access it using global keyword:

last_x = None

def last_in(x):
    global last_x
    return_value, last_x = last_x, x
    return return_value


print(last_in(3), last_in(4), last_in(5), last_in("a"), last_in("12"))

None 3 4 5 a
user_96087590

You can create an attribute for your function to store previous values, e.g. last_in.prev_val. This is somewhat similar to static variables in other languages. You just need to make sure you handle initializing the attribute appropriately. getattr is useful as @Feodoran mentions.

def last_in(x):
    return_val = getattr(last_in, 'prev_val', None)
    last_in.prev_val = x
    return return_val

print(last_in(3), last_in(4), last_in(5), last_in("a"), last_in("12"))

Ouptput:

None 3 4 5 a
user_69425270

Another approach would be to use a Mutable Default Argument:

def last_in(x, prev=[None]):
    prev.append(x)
    return prev[-2]

print(last_in(3),last_in(4),last_in(5), last_in("a"),last_in("12"))

This will produce:

None 3 4 5 a
user_87102650

You can maintain a global list of inputs passed to the function.

valueList=[]
def last_in(x):
    def inner():
        valueList.append(x)
        if(len(valueList)!=1):
            return valueList[len(valueList)-2]
        else:
            return None

    return inner()

print(last_in(3),last_in(4),last_in(5), last_in("a"),last_in("12"))

OUTPUT: None 3 4 5 a

user_86654450

Functions are objects and can have attributes. You can, therefore, store the previous number as an attribute.

def last_in(x):
    if hasattr(last_in, 'last_num'): curr_num = last_in.last_num   
    else: curr_num = None        
    last_in.last_num = x
    return curr_num


print(last_in(2))
print(last_in(3))
print(last_in(4))

Output:

None
2
3
user_135029120

You can do it without global variable:

def last_in(x, last = [None]):
    v, last[0] = last[0], x
    return v

print(last_in(3),last_in(4),last_in(5), last_in("a"),last_in("12"))

Prints:

None 3 4 5 a
user_100359850
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