如何在字典理解中使用if / else?-Python 实用宝典

如何在字典理解中使用if / else?

Python 2.7+中是否存在一种类似于以下内容的方法? { something_if_true if condition else something_if_false for key, value in dict_.items() } 我知道您只要使用'if'就可以做任何事情: { something_if_true for key, value in dict_.items() if condition}

问题:如何在字典理解中使用if / else?

Python 2.7+中是否存在一种类似于以下内容的方法?

{ something_if_true if condition else something_if_false for key, value in dict_.items() }

我知道您只要使用'if'就可以做任何事情:

{ something_if_true for key, value in dict_.items() if condition}

Does there exist a way in Python 2.7+ to make something like the following?

{ something_if_true if condition else something_if_false for key, value in dict_.items() }

I know you can make anything with just 'if':

{ something_if_true for key, value in dict_.items() if condition}

回答 0

您已经知道了:A if test else B是有效的Python表达式。所示的dict理解的唯一问题是dict理解中表达式的位置必须有两个表达式,并用冒号分隔:

{ (some_key if condition else default_key):(something_if_true if condition
          else something_if_false) for key, value in dict_.items() }

final if子句充当过滤器,这与具有条件表达式不同。

You've already got it: A if test else B is a valid Python expression. The only problem with your dict comprehension as shown is that the place for an expression in a dict comprehension must have two expressions, separated by a colon:

{ (some_key if condition else default_key):(something_if_true if condition
          else something_if_false) for key, value in dict_.items() }

The final if clause acts as a filter, which is different from having the conditional expression.


回答 1

@Marcin的答案涵盖了所有内容,但是如果有人想看一个实际的示例,我在下面添加两个:

假设您有以下集合的字典

d = {'key1': {'a', 'b', 'c'}, 'key2': {'foo', 'bar'}, 'key3': {'so', 'sad'}}

并且您想创建一个新字典,其字典中的键指示值中是否包含字符串,'a'则可以使用

dout = {"a_in_values_of_{}".format(k) if 'a' in v else "a_not_in_values_of_{}".format(k): v for k, v in d.items()}

产生

{'a_in_values_of_key1': {'a', 'b', 'c'},
 'a_not_in_values_of_key2': {'bar', 'foo'},
 'a_not_in_values_of_key3': {'sad', 'so'}}

现在假设您有两个这样的字典

d1 = {'bad_key1': {'a', 'b', 'c'}, 'bad_key2': {'foo', 'bar'}, 'bad_key3': {'so', 'sad'}}
d2 = {'good_key1': {'foo', 'bar', 'xyz'}, 'good_key2': {'a', 'b', 'c'}}

如果您要替换的键中d1d2值相同,则可以

# here we assume that the values in d2 are unique
# Python 2
dout2 = {d2.keys()[d2.values().index(v1)] if v1 in d2.values() else k1: v1 for k1, v1 in d1.items()}

# Python 3
dout2 = {list(d2.keys())[list(d2.values()).index(v1)] if v1 in d2.values() else k1: v1 for k1, v1 in d1.items()}

这使

{'bad_key2': {'bar', 'foo'},
 'bad_key3': {'sad', 'so'},
 'good_key2': {'a', 'b', 'c'}}

@Marcin's answer covers it all, but just in case someone wants to see an actual example, I add two below:

Let's say you have the following dictionary of sets

d = {'key1': {'a', 'b', 'c'}, 'key2': {'foo', 'bar'}, 'key3': {'so', 'sad'}}

and you want to create a new dictionary whose keys indicate whether the string 'a' is contained in the values or not, you can use

dout = {"a_in_values_of_{}".format(k) if 'a' in v else "a_not_in_values_of_{}".format(k): v for k, v in d.items()}

which yields

{'a_in_values_of_key1': {'a', 'b', 'c'},
 'a_not_in_values_of_key2': {'bar', 'foo'},
 'a_not_in_values_of_key3': {'sad', 'so'}}

Now let's suppose you have two dictionaries like this

d1 = {'bad_key1': {'a', 'b', 'c'}, 'bad_key2': {'foo', 'bar'}, 'bad_key3': {'so', 'sad'}}
d2 = {'good_key1': {'foo', 'bar', 'xyz'}, 'good_key2': {'a', 'b', 'c'}}

and you want to replace the keys in d1 by the keys of d2 if there respective values are identical, you could do

# here we assume that the values in d2 are unique
# Python 2
dout2 = {d2.keys()[d2.values().index(v1)] if v1 in d2.values() else k1: v1 for k1, v1 in d1.items()}

# Python 3
dout2 = {list(d2.keys())[list(d2.values()).index(v1)] if v1 in d2.values() else k1: v1 for k1, v1 in d1.items()}

which gives

{'bad_key2': {'bar', 'foo'},
 'bad_key3': {'sad', 'so'},
 'good_key2': {'a', 'b', 'c'}}

回答 2

如果您有不同的条件来评估键和值,@ Marcin的答案就是解决方法。

如果键和值的条件相同,则最好在生成器表达式中构建(键,值)元组并馈入dict()

dict((modify_k(k), modify_v(v)) if condition else (k, v) for k, v in dct.items())

它更易于阅读,并且每个键值仅对条件进行一次评估。

借用@Cleb的集合字典的示例:

d = {'key1': {'a', 'b', 'c'}, 'key2': {'foo', 'bar'}, 'key3': {'so', 'sad'}}

假设您只想在其后缀keys,并且在这种情况下,您希望将其替换为集合的长度。否则,键值对应保持不变。avaluevalue

dict((f"{k}_a", len(v)) if "a" in v else (k, v) for k, v in d.items())
# {'key1_a': 3, 'key2': {'bar', 'foo'}, 'key3': {'sad', 'so'}}

In case you have different conditions to evaluate for keys and values, @Marcin's answer is the way to go.

If you have the same condition for keys and values, you're better off with building (key, value)-tuples in a generator-expression feeding into dict():

dict((modify_k(k), modify_v(v)) if condition else (k, v) for k, v in dct.items())

It's easier to read and the condition is only evaluated once per key, value.

Example with borrowing @Cleb's dictionary of sets:

d = {'key1': {'a', 'b', 'c'}, 'key2': {'foo', 'bar'}, 'key3': {'so', 'sad'}}

Assume you want to suffix only keys with a in its value and you want the value replaced with the length of the set in such a case. Otherwise, the key-value pair should stay unchanged.

dict((f"{k}_a", len(v)) if "a" in v else (k, v) for k, v in d.items())
# {'key1_a': 3, 'key2': {'bar', 'foo'}, 'key3': {'sad', 'so'}}

回答 3

在字典理解中使用if / else的另一个示例

我正在为自己的办公室工作在数据输入桌面应用程序上,这种数据输入应用程序通常会从输入小部件中获取所有条目并将其转储到字典中,以进行进一步的处理,例如验证或编辑,我们必须将其返回选择的数据从文件返回到条目小部件等

第一轮使用传统编码(8行):

entries = {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}

a_dic, b_dic = {}, {}

for field, value in entries.items():
    if field == 'ther':
        for k,v in value.items():
            b_dic[k] = v
        a_dic[field] = b_dic
    else:
        a_dic[field] = value
    
print(a_dic)
 {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}”

第二轮我尝试使用字典理解,但是循环仍然存在(6行):

entries = {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}

for field, value in entries.items():
    if field == 'ther':
        b_dic = {k:v for k,v in value.items()}
        a_dic[field] = b_dic
    else:
        a_dic[field] = value
    
print(a_dic)
 {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}”

最后,使用单行字典理解语句(1行):

entries = {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}

a_dic = {field:{k:v for k,v in value.items()} if field == 'ther' 
        else value for field, value in entries.items()}
    
print(a_dic)
 {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}”

我使用python 3.8.3

Another example in using if/else in dictionary comprehension

I am working on data-entry desktop application for my own office work, and it is common for such data-entry application to get all entries from input widget and dump it into a dictionary for further processing like validation, or editing which we must return selected data from file back to entry widgets, etc.

The first round using traditional coding (8 lines):

entries = {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}

a_dic, b_dic = {}, {}

for field, value in entries.items():
    if field == 'ther':
        for k,v in value.items():
            b_dic[k] = v
        a_dic[field] = b_dic
    else:
        a_dic[field] = value
    
print(a_dic)
“ {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}”

Second round I tried to use dictionary comprehension but the loop still there (6 lines):

entries = {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}

for field, value in entries.items():
    if field == 'ther':
        b_dic = {k:v for k,v in value.items()}
        a_dic[field] = b_dic
    else:
        a_dic[field] = value
    
print(a_dic)
“ {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}”

Finally, with a one-line dictionary comprehension statement (1 line):

entries = {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}

a_dic = {field:{k:v for k,v in value.items()} if field == 'ther' 
        else value for field, value in entries.items()}
    
print(a_dic)
“ {'name': 'Material Name', 'maxt': 'Max Working Temperature', 'ther': {100: 1.1, 200: 1.2}}”

I use python 3.8.3


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