Documenting the difference betwen df[!, :col] and df[:, :col]

[aminya]

Fixes #1999

[nalimilan]

Thanks. I've made some changes, let me know what you think.

[scls19fr]

Maybe a good place to warn about the difference could be also in the warning

Warning: `getindex(df::DataFrame, col_ind::ColumnIndex)` is deprecated, use `df[!, col_ind]` 

which could be changed for

Warning: `getindex(df::DataFrame, col_ind::ColumnIndex)` is deprecated, use `df[!, col_ind]` (reference) or `df[:, col_ind]` to get a copy

when user are doing df[:column_name]

[aminya]

Maybe a good place to warn about the difference could be also in the warning

Warning: `getindex(df::DataFrame, col_ind::ColumnIndex)` is deprecated, use `df[!, col_ind]` 

which could be changed for

Warning: `getindex(df::DataFrame, col_ind::ColumnIndex)` is deprecated, use `df[!, col_ind]` (reference) or `df[:, col_ind]` to get a copy

when user are doing df[:column_name]

: is not deprecated. They are two different methods. The whole point of having : is familiarity with Base Arrays! Why should we deprecate such a nice syntax?

[nalimilan]

I've slightly edited the wording.

: is not deprecated.

I don't think that's what @scls19fr suggested. @scls19fr Can you file a new PR? You can edit the file directly on GitHub if you want.

[scls19fr]

Absolutely @nalimilan what I'm saying is that df[:col] is deprecated and df[!, :col] or df[:, :col] should be used instead (and ! or : should be used depending if user want a reference or a copy of column data)

I've been searching for this depwarn using
https://github.com/JuliaData/DataFrames.jl/search?q=use+%60df%5B%21%2C+col_ind%5D%60&unscoped_q=use+%60df%5B%21%2C+col_ind%5D%60

but couldn't find where it's located.

@nalimilan could you point me to code where it's located?

I will submit a PR also

[nalimilan]

It's here: https://github.com/JuliaData/DataFrames.jl/blob/9b30a99303414839579510c4c3138752333fe446/src/deprecated.jl#L1390:L1396

[scls19fr]

Hum... not so easy to add information there I thought it could be done using a simple string... but that's not the case (or at least I don't know how to provide deprecated macro an informative string)

[nalimilan]

You have to replace this with a normal function which will call Base.depwarn. See examples in the file.

[bkamins]

Looks good to me. @nalimilan - can you please merge this if you are OK with the current wording?