[Python-ideas] Allow a group by operation for dict comprehension

28 Jun 2018

      PyToolz, Pandas, Dask .groupby()

toolz.itertoolz.groupby does this succinctly without any
new/magical/surprising syntax.

https://toolz.readthedocs.io/en/latest/api.html#toolz.itertoolz.groupby
...
From https://github.com/pytoolz/toolz/blob/master/toolz/itertoolz.py :
"""
def groupby(key, seq):
    """ Group a collection by a key function
    >>> names = ['Alice', 'Bob', 'Charlie', 'Dan', 'Edith', 'Frank']
    >>> groupby(len, names)  # doctest: +SKIP
    {3: ['Bob', 'Dan'], 5: ['Alice', 'Edith', 'Frank'], 7: ['Charlie']}
    >>> iseven = lambda x: x % 2 == 0
    >>> groupby(iseven, [1, 2, 3, 4, 5, 6, 7, 8])  # doctest: +SKIP
    {False: [1, 3, 5, 7], True: [2, 4, 6, 8]}
    Non-callable keys imply grouping on a member.
    >>> groupby('gender', [{'name': 'Alice', 'gender': 'F'},
    ...                    {'name': 'Bob', 'gender': 'M'},
    ...                    {'name': 'Charlie', 'gender': 'M'}]) #
doctest:+SKIP
    {'F': [{'gender': 'F', 'name': 'Alice'}],
     'M': [{'gender': 'M', 'name': 'Bob'},
           {'gender': 'M', 'name': 'Charlie'}]}
    See Also:
        countby
    """
    if not callable(key):
        key = getter(key)
    d = collections.defaultdict(lambda: [].append)
    for item in seq:
        d[key(item)](item)
    rv = {}
    for k, v in iteritems(d):
        rv[k] = v.__self__
    return rv
"""

If you're willing to install Pandas (and NumPy, and ...), there's
pandas.DataFrame.groupby:

https://pandas.pydata.org/pandas-docs/stable/generated/
pandas.DataFrame.groupby.html

https://github.com/pandas-dev/pandas/blob/v0.23.1/pandas/
core/generic.py#L6586-L6659

Dask has a different groupby implementation:
https://gist.github.com/darribas/41940dfe7bf4f987eeaa#
file-pandas_dask_test-ipynb

https://dask.pydata.org/en/latest/dataframe-api.html#dask.dataframe.DataFram...

On Thursday, June 28, 2018, Chris Barker via Python-ideas <
python-ideas@python.org> wrote:
...
On Thu, Jun 28, 2018 at 8:25 AM, Nicolas Rolin 
wrote:
...
I use list and dict comprehension a lot, and a problem I often have is to
do the equivalent of a group_by operation (to use sql terminology).
I don't know from SQL, so "group by" doesn't mean anything to me, but this:
...
For example if I have a list of tuples (student, school) and I want to
have the list of students by school the only option I'm left with is to
write
student_by_school = defaultdict(list)
    for student, school in student_school_list:
        student_by_school[school].append(student)
seems to me that the issue here is that there is not way to have a
"defaultdict comprehension"
I can't think of syntactically clean way to make that possible, though.
Could itertools.groupby help here? It seems to work, but boy! it's ugly:
In [*45*]: student_school_list
Out[*45*]:
[('Fred', 'SchoolA'),
('Bob', 'SchoolB'),
('Mary', 'SchoolA'),
('Jane', 'SchoolB'),
('Nancy', 'SchoolC')]
In [*46*]: {a:[t[0] *for* t *in* b] *for* a,b *in* groupby(sorted
(student_school_list, key=*lambda* t: t[1]), key=*lambda* t: t[
...: 1])}
...:
...:
...:
...:
...:
...:
...:
Out[*46*]: {'SchoolA': ['Fred', 'Mary'], 'SchoolB': ['Bob', 'Jane'],
'SchoolC': ['Nancy']}
-CHB
--
Christopher Barker, Ph.D.
Oceanographer
Emergency Response Division
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[Python-ideas] Allow a group by operation for dict comprehension

Wes Turner