循环列表中的列表

Question

I have two lists with numbers.

newlist      = [506.5, 133.0, 11104.2]
totalcolumns = [9.2, 10024.5, 610.0, 1100.0]

I want to loop over both lists and format the numbers in the same way:

myformatlists = [newlist, totalcolumns]

for i in range(0,len(myformatlists)):
     myformatlists[i] = ['{0:,}'.format(x) for x in myformatlists[i]]
     myformatlists[i] = [regex.sub("\.0?$", "", x).replace(".", "_").replace(",", ".")

printing

print(str(myformatlists[i]) 

gives the correct new values

but

print(str(newlist))
print(str(totalcolumns))

still gives the old lists.
Why doesn't my for-loop assign the values to the listname in myformatlists[i]?

How can I assign the output of the for-loop to the list in myformatlists?

Answer 1

myformatlists[i] = ['{0:,}'.format(x) for x in myformatlists[i]]

rebinds myformatlists[i] , it does not alter the original item of myformatlists .

You can perform an inplace update of myformatlists[i] using slice notation:

myformatlists[i][:] = ['{0:,}'.format(x) for x in myformatlists[i]]

This will mutate the original list.

But note that there is a problem with the re code where x is not defined becaue the list comprehension is incomplete:

myformatlists[i] = [regex.sub("\.0?$", "", x).replace(".", "_").replace(",", ".")

It should be re.sub and perhaps the rest should be:

myformatlists[i] = [re.sub("\.0?$", "", x).replace(".", "_").replace(",", ".") for x in myformatlists[i]]