Python中的str（）方法适用于所有类型的参数吗？

Question

I am trying to raise error messages using the the following line of code:

def raise_error(value):
    error = "The value " + str(value) + " does not satisfy the given conditions."
    # Other stuff

The value is basically user input obtained from a JSON API, so I am trying to consider all possibilities. Can the method above itself cause an exception in my system if the value given cannot be turned into a string using the str() method? If yes, then what are such values that would create the exception?

Answer 1

The str method will always produce an output, it just might not be meaningful. As noted in the docs here it's calling either __str__ or __repr__ depending on what is defined. For all of the basic types like int, float, bool, etc you'll get a nice output. For a custom class though you'll probably get some garbage like:

class test:
    pass

print(str(test()))

>>> <__main__.test object at 0x107f2ba20>

Now if you define a __str__ method you'll get something nice like so

class test:
    def __str__(self):
        return "A test object"

print(str(test()))
>>> 'A test object'

Answer 2

Short answer: you don't have to worry about it. The string is generated by the __str__ or __repr__ functions, and one or both of these will automatically be created on all new objects and primitives.

---

But technically there are a few scenarios where developers can do dumb stuff.

Scenario A) The __str__ function does not return a string:

class Test():
    def __str__(value):
        return

raise_error(Test())

This results in the error

TypeError: __str__ returned non-string (type NoneType)

Scenario B) The __str__ function has the wrong amount of arguments:

class Test():
    def __str__():
        return 0

raise_error(Test())

This results in the error

TypeError: __str__() takes 0 positional arguments but 1 was given

Obviously in all three of these scenarios the fault lies with whatever developer was creating the bad __str__ functions. I think we can all agree that these are not cases your code should have to handle.