CODEWITHSUNDEEP
pythonfor-loop
SModi
SModi

Reputation: 125

Python for loop purpose of underscore

I'm picking up someone's code. I'm used to writing for loops. But don't understand the purpose of the underscore in this code.

for _, row in dfDD.iterrows():
   row = row.dropna() #drop empty cols
   if len(row)==1:
       continue #because no operation is defined

   frow = row.filter(like=agflag)
   if not frow.empty:
       *code continues*

I looked at the responses provided here:What is the purpose of the single underscore "_" variable in Python? and here:Underscore _ as variable name in Python

In the above case row is already a variable that's being used, so then what's the need for a throwaway variable?

Have removed the rest of the function code to keep things simple, but can provide if needed. Thanks.

Upvotes: 0

Views: 1441

Answers (3)

Eugene Naydenov
Eugene Naydenov

Reputation: 7295

Underscore is the same valid variable name as any other name like item, x, username, etc. But it's more like a convention between programmers to throw unused data into the variable with the meaningless name, which indicates that the data in it is also meaningless in a particular context and doesn't matter.

Such a convention of _ variable name usage could be also seen in unpacking (destructuring) code, for example:

items = [1, 2, 3, 4]
first, *_, last = items

This gives you 1, and 4 as first and last, and everything in between is stored into a meaningless list of [2, 3]. The reader of such code understands that those items in _ don't matter in a given context.

Upvotes: 2

Telcrome
Telcrome

Reputation: 369

Dataframe.iterrows() iterates over tuples (index, row). The index is not used in this case, thus it is ignored.

Try to give the variable a name and set a breakpoint to see what's inside.

Upvotes: 2

AKX
AKX

Reputation: 169269

iterrows() doesn't iterate over only rows, it

Iterate[s] over DataFrame rows as (index, Series) pairs.

There's no other succinct syntax for grabbing the 2nd (or nth) element of each tuple in an iterable of tuples, so it's customary to unpack these in the for loop and throw the unnecessary data into a _ throwaway variable.

Upvotes: 3

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