Print a calendar!

my_calendar.py

#!/usr/bin/env python3
import itertools as it
import operator as op
from datetime import date, timedelta
import calendar as cal


def dates_in_year(year):
    day = timedelta(days=1)
    curr = date(year, 1, 1)
    last = date(year + 1, 1, 1)
    while curr != last:
        yield curr
        curr = curr + day

def grouper(iterable, n, fillvalue=None):
    "Collect data into fixed-length chunks or blocks"
    args = [iter(iterable)] * n
    return it.zip_longest(*args, fillvalue=fillvalue)

def peek_first(iterable):
    first = next(iterable)
    return first, it.chain([first], iterable)

def year_to_months(year):
    grouped_months = it.groupby(year, op.attrgetter('month'))
    return (month for num_, month in grouped_months)

def month_to_weeks(month):
    grouped_weeks = it.groupby(month, lambda d: d.isocalendar()[1])
    return (week for _, week in grouped_weeks)

def format_week(week):
    first, week = peek_first(week)
    day = first.isoweekday()
    left_padded = ' ' * 3 * day + ''.join('%3d' % n.day for n in week)
    both_padded = left_padded.ljust(24)
    return both_padded

def center(width, word):
    return word.center(width - 3).rjust(width)

def layout_month(month):
    first_day, month = peek_first(month)
    month_name = cal.month_name[first_day.month]
    centered = center(24, month_name)
    weeks = [format_week(week) for week in month_to_weeks(month)]
    yield centered
    yield from weeks
    yield from it.repeat(' ' * 24, 6 - len(weeks))

def layout_months(months):
    return (layout_month(m) for m in months)


class Pipe:
    def __init__(self, obj):
        self.obj = obj

    def __or__(self, fun):
        self.obj = fun(self.obj)
        return self

    def __iter__(self):
        return self.obj


if __name__ == '__main__':
    nice_months = Pipe(2016) | dates_in_year | year_to_months | layout_months
    # the usual problem with groups and sharing storage
    # probably should move the list somewhere up the chain
    for quarter in grouper(map(list, nice_months), 3):
        lines = (''.join(x) for x in zip(*quarter))
        print('\n'.join(lines))

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