Python - 如何修复“ValueError: not enough values to unpack (expected 2, got 1)”错误

Question

我需要编写一个函数add_to_dict(d, key_value_pairs)，它将每个给定的键/值对添加到Python字典中。参数key_value_pairs将是一个元组的列表，形式为(key, value)。

该函数应返回所有已更改的键/值对的列表（及其原始值）。

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

我不断收到一个错误

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

我该如何解决这个错误?

- Hamish

---

请添加一个脚本，调用add_to_dict()函数。 - user2575725

for x in dct 只是遍历字典的键吗？ - Ulrich Schwarz

你能指出错误发生在哪一行，并提供一个可运行的函数调用示例吗？即一个 m.c.v.e。 - Paul Rooney

将 d 改为 d.items()。 - eyllanesc

@UlrichSchwarz：是的，确实如此。因此出现了错误。 - dan04

Answer 1

如何调试你的代码

'''
@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

如果您想了解如何正确遍历字典对象，请继续阅读剩余部分。

---

遍历字典对象的不同方法

Python 3.x

以下内容演示了如何在Python 3.x中遍历字典。

迭代键集合

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)

遍历键值对集合

for key, value in d.items():
    print(key, value)

遍历值集合

for value in d.values():
    print(value)

Python 2.x

以下内容演示了如何在Python 2.x中遍历dict。

迭代键的集合

for key in d:
    value = d[key]
    print(key, value)

keys() 返回字典 d 的键集合的列表

for key in d.keys():
    value = d[key]
    print(key, value)

iterkeys() 返回字典 d 的键集的一个迭代器

for key in d.iterkeys():
    value = d[key]
    print(key, value)

遍历键值对集合

values() 返回字典 d 的键值对集合，类型为列表

for key, value in d.items():
    print(key, value)

itervalues() 返回字典 d 的键值对集合的迭代器

for key, value in d.iteritems():
    print(key, value)

遍历值集合

values() 返回字典 d 的值集合的列表。

for value in d.values():
    print(value)

itervalues() 返回字典 d 的值集合的迭代器。

for value in d.itervalues():
    print(value)

参考资料：

• Python中列表和迭代器的区别是什么？

- KaiserKatze

Answer 2

使用 items() 方法来解决，例如：

d = {"foo": "bar"}

for key, value in d.items():
    print key, value

- pfctgeorge

Answer 3

如果您不遍历包含100万条目的字典，而只是遍历可能更改的列表，并查看它是否会更改字典，则可以避免此错误：

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

输出：

[] # 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

您还可以在参数上加入一些验证 - 如果任何参数有误，则不会执行任何操作，并出现一个语音错误：

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

这样您就可以获得更清晰的错误信息:

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

- Patrick Artner