Python ValueError too many values to unpack

Question

I am getting that exception from this code   class Transaction      def   init    self           self materials           def add material  self  m           self materials m type   m purity    m      def serialize  self           ser str    transaction start n           for k  m in self materials              ser str    m serialize             sert     transaction end n          return ser str   The for line is the one throwing the exception  The ms are Material objects  Anybody have any ideas why

User · Accepted Answer

self materials is a dict and by default you are iterating over just the keys  which are strings    Since self materials has more than two keys   they can t be unpacked into the tuple  k  m   hence the ValueError exception is raised   In Python 2 x  to iterate over the keys and the values  the tuple  k  m    we use self materials iteritems     However  since you re throwing the key away anyway  you may as well simply iterate over the dictionary s values   for m in self materials itervalues      In Python 3 x  prefer dict values    which returns a dictionary view object    for m in self materials values

User · Answer

for k  m in self materials items      example   miles dict     Monday  1   Tuesday  2 3   Wednesday  3 5   Thursday  0 9  for k  v in miles dict items        print   s   s     k  v

User · Answer

Iterating over a dictionary object itself actually gives you an iterator over its keys  Python is trying to unpack keys  which you get from m type   m purity into  m  k    My crystal ball says m type and m purity are both strings  so your keys are also strings  Strings are iterable  so they can be unpacked  but iterating over the string gives you an iterator over its characters  So whenever m type   m purity is more than two characters long  you have too many values to unpack   And whenever it s shorter  you have too few values to unpack    To fix this  you can iterate explicitly over the items of the dict  which are the  key  value  pairs that you seem to be expecting  But if you only want the values  then just use the values    In 2 x  itervalues  iterkeys  and iteritems are typically a better idea  the non-iter versions create a new list object containing the values keys items  For large dictionaries and trivial tasks within the iteration  this can be a lot slower than the iter versions which just set up an iterator

[python] Python ValueError: too many values to unpack

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