Python - How to fix “ValueError: not enough values to unpack (expected 2, got 1)”
use items() to resolve, like:
d = {"foo": "bar"}
for key, value in d.items():
print key, value
How you should debug your code
'''
@param d: a dictionary
@param key_value_pairs: a list of tuples in the form `(key, value)`
@return: a list of tuples of key-value-pair updated in the original dictionary
'''
def add_to_dict(d, key_value_pairs):
newlist = []
for pair in key_value_pairs:
# As is mentioned by Mr Patrick
# you might not want to unpack the key-value-pair instantly
# to avoid possible corrupted data input from
# argument `key_value_pairs`
# if you can't guarantee its integrity
try:
x, y = pair
except (ValueError):
# unable to unpack tuple
tuple_length = len(pair)
raise RuntimeError('''Invalid argument `key_value_pairs`!
Corrupted key-value-pair has ({}) length!'''.format(tuple_length))
# Instead of using nesting loop
# using API would be much more preferable
v = d.get(x)
# Check if the key is already in the dictionary `d`
if v:
# You probably mean to append a tuple
# as `array.append(x)` takes only one argument
# @see: https://docs.python.org/3.7/library/array.html#array.array.append
#
# Besides, hereby I quote
# "The function should return a list of all of the key/value pairs which have changed (with their original values)."
# Thus instead of using the following line:
#
# newlist.append((x, y,))
#
# You might want a tuple of (key, old_value, new_value)
# Hence:
newlist.append((x, v, y,))
# I don't know if you want to update the key-value-pair in the dictionary `d`
# take out the following line if you don't want it
d[x] = y
return newlist
Please keep reading the remaining part if you want to know how to traverse a dict object properly.
Different ways to traverse a dict object
Python 3.x
The following segments demonstrate how to traverse a dict in Python 3.x.
Iterate the set of keys
for key in d:
value = d[key]
print(key, value)
the code segment above has the same effect as the following one:
for key in d.keys():
value = d[key]
print(key, value)
Iterate the set of key-value-pairs
for key, value in d.items():
print(key, value)
Iterate the set of values
for value in d.values():
print(value)
Python 2.x
The following segments demonstrate how to traverse a dict in Python 2.x.
Iterate the set of keys
for key in d:
value = d[key]
print(key, value)
keys() returns a list of the key set of dictionary d
for key in d.keys():
value = d[key]
print(key, value)
iterkeys() returns an iterator of the key set of dictionary d
for key in d.iterkeys():
value = d[key]
print(key, value)
Iterate the set of key-value-pairs
values() returns a list of the key-value-pair set of dictionary d
for key, value in d.items():
print(key, value)
itervalues() returns an iterator of the key-value-pair set of dictionary d
for key, value in d.iteritems():
print(key, value)
Iterate the set of values
values() returns a list of the value set of dictionary d
for value in d.values():
print(value)
itervalues() returns a iterator of the value set of dictionary d
for value in d.itervalues():
print(value)
Reference:
- What is the difference between list and iterator in Python?
You can avoid this error if you do not 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 could also throw in some validation onto your parameters - if any parameter is wrong, nothing will be executed and an speaking error arises:
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
So you get 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