Python 3: dict & dict.keys()

Neil Cerutti neilc at
Wed Jul 24 14:54:36 CEST 2013

On 2013-07-24, Peter Otten <__peter__ at> wrote:
>> So, my question boils down to:  in Python 3 how is dict.keys()
>> different from dict?  What are the use cases?
> I just grepped through /usr/lib/python3, and could not identify
> a single line where some_object.keys() wasn't either wrapped in
> a list (or set, sorted, max) call, or iterated over.
> To me it looks like views are a solution waiting for a problem.

Here's a case of using keys as a set-like view from my own
"production" code (i.e., I used it once for one important job):

seen = set()
students = {}
dates = []

for fname in sorted(glob.glob("currentterm201320?.csv")):
    print(fname, end="\n\t")
    date = get_date(fname)
    term = fname[-11:-4]
    r = reg.registration(term, path=".")
    regs = r.keys()
    for alt_id in regs & seen:
    for alt_id in seen - regs:
    for alt_id in regs - seen:
        students[alt_id] = [None]*(len(dates)-1) + [r[alt_id]]

It was a very nice way to to do three different things depending
on the student sin the set I was working with, compared to a
registration list:

Granted the line was originally "regs = set(regs.keys())" before
it occurred to me that it sucked to take what must be equivalent
to a set, convert to a list, and then back to set again.

Thanks to the set-like view of dict.keys it worked just like one
might hope.

Looking at it again "seen" might be a redundant parallel version
of students.keys().

Neil Cerutti

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