I was stuck on a problem today using an open statement where I was trying to open a file for writing

e.g. 

    with open("RESULTS.CSV", "W") as csvfile:
        csvwriter = csv.writer(csvfile)
        csvwriter.writerow(["X", "Y"])
        csvwriter.writerows(data) 

I did not notice I had the mode W in upper case. I am not sure if there is a legacy reason for only allowing lower case arguments here but I think a quick note in the documentation that it's case sensitive or a check (or note) when throwing an error would be helpful?  such as 

    ValueError: invalid mode: 'W' -- your case appears to be an upper case, please ensure the case of the mode is correct

or 

    ValueError: invalid mode: 'W' -- note the mode is case sensitive

could be helpful?

Although I'm not sure the existing message needs changing, if a change is going to be made I think listing the valid option characters might be more useful.

I also think the current error message and docs are clear enough. Things are usually case sensitive, and we don't state that in docs. 

Specifying the list of modes in the error message is possible, but I expect that we will get complaints about variations we did not list (why do you mention only "r" and "w" when "rw" is also fine?)