Home/ Questions/How to fix the issue: ''not enough values to unpack (expected 2, got 1)''?
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ozellahammes
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Asked: 2022-07-15T04:55:01+00:00 In:

How to fix the issue: ”not enough values to unpack (expected 2, got 1)”?

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I get the ”not enough values to unpack (expected 2, got 1)” error when i run my program.

In order to add each key/value pair to a Python dictionary, I need to create the function add_to_dict(d, key_value_pairs). A list of tuples with the type key_value_pairs will make up the argument (key, value).

A list of all the key/value pairs that have changed should be returned by the function (with their original values).

def add_to_dict(d, key_value_pairs):

   newlist = []

   for key,value in d:
       for x,y in key_value_pairs:
           if x == key:
              newlist.append(x,y)

return newlist

And then I get the error message:

ValueError: not enough values to unpack (expected 2, got 1)

Can someone give me some advice?

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1 Answer

  1. Best Answer
    Expert

    The cause: I think this error happens because you iterated over the dict (of 1 million entries).

    Solution: To fix this error, you can’t iterate over the dict (of 1 million entries) but only over the list of possible changes and see if it changes anything in the dict:

    def add_to_dict(d, key_value_pairs):
    """Adds all tuples from key_value_pairs as key:value to dict d,
    returns list of tuples of keys that got changed as (key, old value)"""
    newlist = []


    for item in key_value_pairs:

        # this handles your possible unpacking errors
        # if your list contains bad data
        try:
           key, value = item
       except (TypeError,ValueError):
           print("Unable to unpack {} into key,value".format(item))

       # create entry into dict if needed, else gets existing
       entry = d.setdefault(key,value)
    
       # if we created it or it is unchanged this won't execute
       if entry != value:
           # add to list
           newlist.append( (key, entry) )
           # change value
           d[key] = value

   return newlist

d = {}
print(add_to_dict(d, ( (1,4), (2,5) ) )) # ok, no change
print(add_to_dict(d, ( (1,4), (2,5), 3 ) )) # not ok, no changes
print(add_to_dict(d, ( (1,7), (2,5), 3 ) )) # not ok, 1 change

    Output:

    [] # ok

Unable to unpack 3 into key,value
[] # not ok, no change

Unable to unpack 3 into key,value
[(1, 4)] # not ok, 1 change

    You might also add some parameter validation; if any parameter is incorrect, nothing will be done and a spoken error would result:

    import collections

def add_to_dict(d, key_value_pairs):
    """Adds all tuples from key_value_pairs as key:value to dict d,
    returns list of tuples of keys that got changed as (key, old value)"""
  
    if not isinstance(d,dict):
        raise ValueError("The dictionary input to add_to_dict(dictionary,list of tuples)) is no dict")

    if not isinstance(key_value_pairs,collections.Iterable):
        raise ValueError("The list of tuples input to add_to_dict(dictionary,list of tuples)) is no list")

    if len(key_value_pairs) > 0:
        if any(not isinstance(k,tuple) for k in key_value_pairs):
            raise ValueError("The list of tuples includes 'non tuple' inputs")
   
        if any(len(k) != 2 for k in key_value_pairs):
            raise ValueError("The list of tuples includes 'tuple' != 2 elements")

    newlist = []
    for item in key_value_pairs:
        key, value = item

        # create entry into dict if needed, else gets existing
        entry = d.setdefault(key,value)

        # if we created it or it is unchanged this won't execute
        if entry != value:
            # add to list
            newlist.append( (key, entry) )
            # change value
            d[key] = value

return newlist

    This results in clearer error messages:

    add_to_dict({},"tata")
# The list of tuples input to add_to_dict(dictionary,list of tuples)) is no list

add_to_dict({},["tata"])
# The list of tuples includes 'non tuple' inputs

add_to_dict({},[ (1,2,3) ])
# The list of tuples includes 'tuple' != 2 elements

add_to_dict({},[ (1,2) ])
# ok
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