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.



In [5]:

    
record = ('Dave', 'dave@example.com', '773-555-1212', '847-555-1212')
name, email, *phone_numbers = record

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









    



Dave
dave@example.com
['773-555-1212', '847-555-1212']

Example 2



In [6]:

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









    



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

Example 3



In [7]:

    
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



In [8]:

    
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



In [9]:

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

print (name)
print (year)









    



ACME
2012