“For” loop first iteration

Question

I would like to inquire if there is an elegant pythonic way of executing some function on the first loop iteration. The only possibility I can think of is:

f         


        

Answer 1

Your question is contradictory. You say "only do something on first iteration", when in fact you are saying do something different on first/subsequent iterations. This is how I would attempt it:

copyfn = root.copy
for member in something.get():
    copyfn(member)
    foo(member)
    copyfn = somewhereElse.copy

Answer 2

I think this is quite elegant, but maybe too convoluted for what it does...

from itertools import chain, repeat, izip
for place, member in izip(chain([root], repeat(somewhereElse)), something.get()):
    place.copy(member)
    foo(member)

Answer 3

Something like this should work.

for i, member in enumerate(something.get()):
    if i == 0:
         # Do thing
    # Code for everything

However, I would strongly recommend thinking about your code to see if you really have to do it this way, because it's sort of "dirty". Better would be to fetch the element that needs special handling up front, then do regular handling for all the others in the loop.

The only reason I could see for not doing it this way is for a big list you'd be getting from a generator expression (which you wouldn't want to fetch up front because it wouldn't fit in memory), or similar situations.

Answer 4

If something.get() iterates over something, you can do it also as follows:

root.copy(something.get())

for member in something.get():
  #  the rest of the loop

Answer 5

This works:

for number, member in enumerate(something.get()):
    if not number:
        root.copy(member)
    else:
        somewhereElse.copy(member)
    foo(member)

In most cases, though, I'd suggest just iterating over whatever[1:] and doing the root thing outside the loop; that's usually more readable. Depends on your use case, of course.

Answer 6

Here is what works for me

    dup_count = 0
    for x in reversed(dup_list):
        dup_count += 1
        if dup_count == 1:
            print("First obj {}: {}".format(dup_count,x))
        else:
            print("Object # {}:  {}".format( dup_count,x  ))