tuples as records and immutable lists

double_duty_tuples.py

#!/usr/bin/env python3
# tuples as records & immutable lists

## TUPLES AS RECORDS ##

lax_coordinates = (33.9425, -118.408056)
city, year, pop, chg, area = ("Tokyo", 2003, 32_450, 0.66, 8014)
traveler_ids = [("USA", "31195855"), ("BRA", "CE342567"), ("ESP", "XDA205856")]

# the % formatting operator understands tuples and treats each item as a separate
# field.
for passport in sorted(traveler_ids):
    print("%s/%s" % passport)
# BRA/CE342567
# ESP/XDA205856
# USA/31195855

# the for loop knows how to retrieve the items of a tuple separately—this is called
# "unpacking." here we are not interested in the second item, so we assign it to _, a
# dummy variable.
for country, _ in traveler_ids:
    print(country)
# USA
# BRA
# ESP

## TUPLES AS IMMUTABLE LISTS ##
# even though immutable, the last item is mutable.
a = (1, "egg", [2, 3])
b = (1, "egg", [2, 3])
print(a == b)
# True

a[-1].append(4)
print(a == b)
# False


# function to explicitly determine whether a tuple has fixed value
def fixed(o):
    try:
        hash(o)
    except TypeError:
        return False
    return True


a = (1, "egg", [2, 3])
b = (1, "egg", (2, 3))

print(fixed(a))
# False
print(fixed(b))
# True