List comprehension ValueError: too many values to unpack

Question

I have a process which generates 2-item lists as [datestamp, timestamp] .

if I put this into a list comprehension as:

[(ds,ts) for ds,ts in process]

my desired outcome is:

[(ds,ts), (ds,ts), (ds,ts)]

What I get is ValueError: too many values to unpack .

This is because the for loop iteration into the list returned by the process:

for ds,ts in [datestamp, timestamp]

does not assign ds=datestamp , ts=timestamp , but rather iterates across each letter ... if this worked it would give (d,t) , (a,i) , (t,m) , (e,e) etc

So I know what's wrong ... but not how to make this work! (and yeah, this does feel silly ... I know the answer will be real simple

Answer 1

This works:

process = [[1,2],[3,4],[5,6]]
a = []
for ds, ts in process:
    print(ds, ts)
    a.append((ds, ts))

As well as

z = [(ds, ts) for (ds, ts) in process]
q = [(ds, ts) for ds, ts in process]

If you are getting a 'ValueError: too many values to unpack' exception, process must be producing iterables that have more than two items.

Unwind the list comprehension and Handle the exception - print something relevant in the except suite to see what might be going wrong, then work backwards to the source. something like

process = [[1,2],[3,4],[5,6], [7,8,9]]

a = []
try:
    for thing in process:
        ds, ts = thing
        a.append((ds, ts))
except ValueError as e:
    print(e, '\t', thing)

Answer 2

You need to debug. There must be some items in your list that are not pairs as you expect.

One way you can find them would be:

problems = [(idx, value) for idx,value in enumerate(process) if len(value)!=2]

This will give you a list of the problem indexes and items in your list.

If that gives an error- for example, a TypeError because the value object has no __len__ method- then change it to:

problems = [(idx, value) for idx,value in enumerate(process) if not hasattr(value, '__len__')]