Unpacking Elements from Iterables of Arbitrary Length

Problem

• You need to unpack N elements from an iterable, but the iterable may be longer than N elements, causing a “too many values to unpack” exception.

Solution

• Python “star expressions” can be used to address this problem.

Example 1

• Suppose you have user records that consist of a name and email address, followed by an arbitrary number of phone numbers.

record = ('Dave', '[email protected]', '773-555-1212', '847-555-1212')
name, email, *phone_numbers = record

print (name)
print (email)
print (phone_numbers)

Dave
[email protected]
['773-555-1212', '847-555-1212']

Example 2

*trailing, current = [10, 8, 7, 1, 9, 5, 10, 3]
print (trailing)
print (current)

[10, 8, 7, 1, 9, 5, 10]
3

Example 3

records = [
         ('foo', 1, 2),
         ('bar', 'hello'),
         ('foo', 3, 4),
    ]
def do_foo(x, y): 
    print('foo', x, y)

def do_bar(s): 
    print('bar', s)

for tag, *args in records: 
    if tag == 'foo':
        do_foo(*args) 
    elif tag == 'bar':
        do_bar(*args)

foo 1 2
bar hello
foo 3 4

Example 4

• Dealing with certain kinds of string processing operations

line = 'nobody:*:-2:-2:Unprivileged User:/var/empty:/usr/bin/false'
uname, *fields, homedir, sh = line.split(':')

print (uname)
print (homedir)
print (sh)

nobody
/var/empty
/usr/bin/false

Example 5

• Unpack values and throw them away

record = ('ACME', 50, 123.45, (12, 18, 2012))
name, *_, (*_, year) = record

print (name)
print (year)

ACME
2012