9.1. Comprehensions Nesting

9.1.1. Nested Comprehensions

DATA = [
    {'lastname': 'Watney'},
    {'firstname': 'Melissa', 'lastname': 'Lewis'},
    {'firstname': 'Rick'},
    {'firstname': 'Beth', 'lastname': 'Johanssen'},
    {'firstname': 'Chris', 'lastname': 'Beck', 'birthdate': 1961},
]

fieldnames = set(key
    for row in DATA
        for key in row.keys()
)

print(fieldnames)
# {'birthdate', 'lastname', 'firstname'}

DATA = [
    {'lastname': 'Watney'},
    {'firstname': 'Melissa', 'lastname': 'Lewis'},
    {'firstname': 'Rick'},
    {'firstname': 'Beth', 'lastname': 'Johanssen'},
    {'firstname': 'Chris', 'lastname': 'Beck', 'birthdate': 1961},
]

fieldnames = set()
fieldnames.update(key
    for row in DATA
        for key in row.keys()
)

print(fieldnames)
# {'birthdate', 'lastname', 'firstname'}

DATA = [
    {'lastname': 'Watney'},
    {'firstname': 'Melissa', 'lastname': 'Lewis'},
    {'firstname': 'Rick'},
    {'firstname': 'Beth', 'lastname': 'Johanssen'},
    {'firstname': 'Chris', 'lastname': 'Beck', 'birthdate': 1961},
]

fieldnames = set()
fieldnames.update(key for row in DATA for key in row.keys())

print(fieldnames)
# {'birthdate', 'lastname', 'firstname'}

9.1.2. Generator comprehensions

DATA = [
    {'lastname': 'Watney'},
    {'firstname': 'Melissa', 'lastname': 'Lewis'},
    {'firstname': 'Rick'},
    {'firstname': 'Beth', 'lastname': 'Johanssen'},
    {'firstname': 'Chris', 'lastname': 'Beck', 'birthdate': 1961},
]

fieldnames = set()

fieldnames.add(key
    for row in DATA
        for key in row.keys()
)

print(fieldnames)
# {<generator object <genexpr> at 0x1179a0a50>}