Good way to iterate lists of different lengths and set default value

In Python, is there a good way to iterate through lists of different lengths?

For example,

a = [1,2,3]

b=[4,5]

c = [a,b]

for val1, val2, val3 in c:

print val1
print val2
print val3


Assuming that the list will have at least 2 values, and in some list, 3rd value is optional. The above for loop didn't work for b, obviously, that val3 is not available for list 'b'. In that case, I want to print the val3 as 0. Can I give a default value in case of unavailability?

for

for val1, val2, val3=0 in c:


The above syntax didn't work either. Please help.

5 Answers
5

If you want to be fancy ("elegant"?), you can pad a given list with zeros:

def pad_list(t, size, default):
return t + [default] * (size - len(t))

for x in c:
v1, v2, v3 = pad_list(x, 3, 0)

print(v1)
print(v2)
print(v3)


Similarly, if you're working with tuples, here's another function:

def pad_list(t, size, default):
return t + (default,) * (size - len(t))


You could use zip_longest with fillvalue handling empty slot for this case:

zip_longest

fillvalue

from itertools import zip_longest

a = [1,2,3]
b = [4,5]

l =
for x, y in zip_longest(a, b, fillvalue=0):
l.append((x, y))

print(list(zip(*l)))
# [(1, 2, 3), (4, 5, 0)]


If you need values out of list, just replace last print with:

print

for val1, val2, val3 in zip(*l):
print(val1)
print(val2)
print(val3)

# 1
# 2
# 3
# 4
# 5
# 0


More succinctly (and generally), zip(*zip_longest(*c, fillvalue=0)) where c = [a, b]. This works with arbitrarily sized c.
– Mateen Ulhaq
2 days ago

zip(*zip_longest(*c, fillvalue=0))

c = [a, b]

c

this is very simple

c = [1,2,3]
val1 , val2 , *val3= [1,2 , 3]


val1 = 1 , val2 =3 , val3=[3]

c=[1,2]
val1 , val2 , *val3= [1,2 , 3]


val1 = 1 , val2 =3 , val3=

def foreach(l):
def deco(f):
for xs in l:
f(*xs)
return deco


@foreach([[1, 2, 3], [4, 5]])
def _(a, b, c=6):
print(a, b, c, sep='n')


The simplest method to concatenate the lists is via chain function from the itertools module.

chain

itertools

import itertools

a = [1, 2, 3, 4, 5, 6]
b = [7, 8, 9, 10, 11, 12]
c = ['A', 'B', 'C', 'D', 'E', 'F', 'U']


combined = itertools.chain( a, b, c ) # combines in order

# enumerate lists to allow for iteration
for index, value in enumerate(combined):
print(value, end = ' ')


1 2 3 4 5 6 7 8 9 10 11 12 A B C D E F U


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