Using lambdas in a function within a function

Question

I have an assignment where I'm asked to modify some code. The original function is this:

def selectivelyCopy(inputFile,outputFile,predicate):
  linesCopied = 0
  for line in inputFile:
    if predicate(line):#test the line with the predicate
        outputFile.write(line)
        linesCopied+=1
inputFile.close()
return linesCopied

Now I am suppose to add the parameter transform, a function that takes in as its parameter a string, and returns a string according to the transformation specified by the user. If transform is omitted from the function call, lines from the input file are written unchanged.

Here is what I have so far:

def selectivelyCopy2(inputFile,outputFile,predicate, transform):
    def transform(x = lambda x: x):
        return(x) 

    linesCopied = 0
    for line in inputFile:
        if predicate(line): #test the line with the predicate
            outputFile.write(line)
            linesCopied+=1
    inputFile.close()
    return linesCopied

I'm not sure where to proceed from here. I think I'm suppose to read the input file line, but write the transformed line...or something?

Try running the code. You're CPU's time is worth less than ours. Also tag as homework if this is homework. — Shep
– Shep, Commented Apr 16, 2012 at 6:25
I did run it. It just runs as if the definition wasn't there. No errors. — kk.lau
– kk.lau, Commented Apr 16, 2012 at 6:38
Sorry for stating the obvious, but ... you run the code above? With nothing extra? Did you call your function? — Veky
– Veky, Commented Aug 7, 2016 at 16:39

steveha · Accepted Answer · 2012-04-16 06:37:52Z

3

Now I am suppose to add the parameter transform, a function that takes in as its parameter a string, and returns a string according to the transformation specified by the user. If transform is omitted from the function call, lines from the input file are written unchanged.

That sounds to me very simple: you are supposed to accept a function argument called transform, and if it is supplied, you call it. If it is not supplied, you either don't call it, or else you call a trivial function that returns its input unchanged.

I suggest that you use a default argument of None for transform. Then check to see if transform is None. If it is not, then try to call it, passing the current line, and collecting the output as the new current line. If transform is None then you just write the current line unchanged.

Alternatively, you could declare this trivial function:

def nop(x):
    return x

And then specify that the default for argument transform is the function nop. Which is better, to test for None and call nothing, or to have a sensible no-operation default function and always call it? I think this is mostly a matter of personal preference. The test for None avoids the overhead of a function call, so it is probably slightly faster, but it probably isn't a big deal either way.

There is no reason to declare a private function named transform, and by doing that you are making it impossible to check what the argument transform is.

answered Apr 16, 2012 at 6:37

steveha

77.1k21 gold badges94 silver badges119 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

Nolen Royalty Over a year ago

You took the words right out of my mouth. Slightly more elegant, but also less inefficient, might be to set transform's default to transform = lambda x: x so that you can you can always call transform instead of checking whether it is None. You definitely hinted at the idea, just thought I'd mention it more explicitly.

steveha Over a year ago

I usually tend to make named functions like nop() and specify them by name, rather use lambda as you showed, either one is fine. Another example that pretty much comes down to personal taste.

kk.lau Over a year ago

Interesting. My professor specifically stated to make another function, but I guess I will just do it another way. Thanks.

Shep Over a year ago

@kk.lau, lambda does make another function, it's just not defined the same way. i.e. f = lambda x: x defines exactly the same function as def f(x): return x. (you can't use the later as a default argument, though)

kk.lau Over a year ago

Yes, I knew this, but for some reason it wasn't processing. Maybe it was the 9 hours of straight homework I've been doing. Thank you :)

Collectives™ on Stack Overflow

Using lambdas in a function within a function

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related