格式化字典键：AttributeError：'字典'对象没有属性'keys（）'

Question

在字符串中格式化字典键的正确方法是什么？

当我这样做：

>>> foo = {'one key': 'one value', 'second key': 'second value'} 
>>> "In the middle of a string: {foo.keys()}".format(**locals()) 

我想到：

"In the middle of a string: ['one key', 'second key']" 

我能得到什么：

Traceback (most recent call last): 
    File "<pyshell#4>", line 1, in <module> 
    "In the middle of a string: {foo.keys()}".format(**locals()) 
AttributeError: 'dict' object has no attribute 'keys()' 

但你可以看到，我的字典具有键：

>>> foo.keys() 
['second key', 'one key'] 

Answer 1

您不能在占位符中调用方法。您可以访问特性和属性，甚至指数的价值 - 但你不能调用方法：

class Fun(object): 
    def __init__(self, vals): 
     self.vals = vals 

    @property 
    def keys_prop(self): 
     return list(self.vals.keys()) 

    def keys_meth(self): 
     return list(self.vals.keys()) 

实例与方法（不及格）：

>>> foo = Fun({'one key': 'one value', 'second key': 'second value'}) 
>>> "In the middle of a string: {foo.keys_meth()}".format(foo=foo) 
AttributeError: 'Fun' object has no attribute 'keys_meth()' 

实例财产（工作）：

>>> foo = Fun({'one key': 'one value', 'second key': 'second value'}) 
>>> "In the middle of a string: {foo.keys_prop}".format(foo=foo) 
"In the middle of a string: ['one key', 'second key']" 

格式化语法表明您只能访问属性（a la getattr）或索引（a la __getitem__）占位符（取自"Format String Syntax"）：

The arg_name can be followed by any number of index or attribute expressions. An expression of the form '.name' selects the named attribute using getattr() , while an expression of the form '[index]' does an index lookup using __getitem__() .

---

使用Python 3.6，你可以轻松地与F-串做到这一点，你甚至不必在locals经过：

>>> foo = {'one key': 'one value', 'second key': 'second value'} 
>>> f"In the middle of a string: {foo.keys()}" 
"In the middle of a string: dict_keys(['one key', 'second key'])" 

>>> foo = {'one key': 'one value', 'second key': 'second value'} 
>>> f"In the middle of a string: {list(foo.keys())}" 
"In the middle of a string: ['one key', 'second key']" 

Answer 2

"In the middle of a string: {}".format(list(foo.keys())) 

Answer 3

"In the middle of a string: {}".format([k for k in foo]) 

Answer 4

正如上面其他人所说的那样，你不能按照你喜欢的方式去做，这里有更多的信息可以参考python string format calling a function