BUG: to_csv handles cols= reordering,dupe cols GH3454

[ghost]

#3454, #3457, #3455

@jreback, comments?

[jreback]

so this is a compat with the original to_csv engine?

[ghost]

Actually it's 3 bugs rolled into one.

for unique col reordering, it's compat yeah. The original test suite for to_csv didn't cover this
case and so we didn't see it.

for dupe columns, engine='python' is totally broken, so trying that now raises. #3457
getitem is broken for dupe columns, #3455, so for now, the new engine will write out
a full frame with dupe cols unless you also try to select a subset in which case it'll raise.

what a PITA.

[jreback]

let's wait a bit, I think can these pretty easily (when they are in the same block, that case is still tricky)

[ghost]

s***, here we go with the RELEASE.rst conflicts again.

[jreback]

yep....I see them evertime I add at the end, if I add in the middle its ok.....?? maybe we should NOT add at the end and just reorder every once in a while?

[ghost]

yeah, insert at a random location, p(collision)-->0 n as we get more tickets in.
same for GH issue links.

[jreback]

#3459 merged in

[jreback]

@y-p dup column changes all merged in.....pls revisit any issues that you think need