I am trying to understand the __str__ method in Python.
class Test:
def __str__(self):
return 5
t = Test()
print(t.__str__())
In this method it returns an integer value but the print method is able to print it.
But, when I tried print(t) it threw the error TypeError: __str__ returned non-string (type int).
As I understand print(t) is also calling the __str__(self) method.
Why didn't print(t.__str__()) want the the string type conversion?
What you’re doing is equivalent to print(5), which works because print calls __str__ on 5 to get a string. But passing the object, print calls __str__ on the object and doesn’t get an actual string in response.
It's all about extra checks that python does with some built-in functions.
len() --> __len__() + some checks
str() --> __str__() + some checks
There is a difference between when "you" call a method explicitly or when that method gets called "by Python"! The point is when Python calls your method it will do some checks for you. (That's one of the reasons that we should use those built-in functions instead of calling relevant dunder method.)
We can see that behavior with len() and __len__() as well:
class Test:
def __len__(self):
return 'foo'
t = Test()
print(t.__len__()) # fine
print(len(t)) # TypeError: 'str' object cannot be interpreted as an integer
So python checked for returning integer in second print statement! that's what expected from __len__().
Same thing happens here. When you call print(t) Python itself calls __str__() method, so it does check to see if __str__() returns a string that is expected or not. (same thing happens with str(t))
But, when you say print(t.__str__()), first, you're calling it's __str__() method on the instance explicitly by yourself, there is no checking here... what does get back ? number 5, and then python will run print(5).
When you call t.__str__() directly it is just like any other method. The method __str__ method was overwritten, so there is nothing special about it when calling it directly.
When doing print(t) invocation happens internally, where some typechecking takes place
if (!PyUnicode_Check(res)) {
_PyErr_Format(tstate, PyExc_TypeError,
"__str__ returned non-string (type %.200s)",
Py_TYPE(res)->tp_name);
Py_DECREF(res);
return NULL;
The manual states:
The return value must be a string object.
so you should do something like
def __str__(self):
return str(5)
or better, something more meaningful like
def __str__(self) -> str:
return "TestObject with id: {}".format(self.id)
(The return type can be added to the function decalaration so your editor will let you know if it doesn't have the right type.)
When you call print(t), print function tries to get the str(t) value which returns integer. The value has to be str, so it raises an exception. But when you call print(t.__str__()), it doesn't raise an exception because the method acts like an ordinary method and the return value type doesn't have to be str.
I will put here some finding that I found while looking at the behaviour of the print function. So, here goes nothing.
Why didn't print(t.str()) want the the string type conversion?
It actually does (but, I think not the way you expect it). As most people have noted here, what happened with the code, is that it will evaluate the __str__ function first (so, you got the number 5 here). And, what happened is that, it does the conversion (if you look at the source code here) using the __str__ of int class (so, your number 5 will be printed as "5")
But, when I tried print(t) it threw the error TypeError: str returned non-string (type int).
This happen because there is a checking to ensure the object "representated" as string properly. This behaviour can be checked on this source code
In theory, you are correct in this part:
As I understand print(t) is also calling the str(self) method.
Inside Python internal, when calling the __str__ method, Python does call the method __str__(self), but only one time, and it does get the result, a number 5:
https://github.com/python/cpython/blob/3.10/Objects/object.c#L499
But then, Python will check the result in C level, reports an error if the result is not a string:
https://github.com/python/cpython/blob/3.10/Objects/object.c#L505
It will not try to call __str__ method on the result again. So instead see the result, you will get the error TypeError: __str__ returned non-string (type int).
"print" implicitly performs string conversion so you get the same output either way.
Special Methods
In Python, certain special names are invoked by the Python interpreter in special circumstances. For instance, the init method of a class is automatically invoked whenever an object is constructed. The str method is invoked automatically when printing, and repr is invoked in an interactive session to display values.
There are special names for many other behaviors in Python. Some of those used most commonly are described below.
True and false values. We saw previously that numbers in Python have a truth value; more specifically, 0 is a false value and all other numbers are true values. In fact, all objects in Python have a truth value. By default, objects of user-defined classes are considered to be true, but the special bool method can be used to override this behavior. If an object defines the bool method, then Python calls that method to determine its truth value.
Let’s try literals of different built-in types and see what comes out:
print(42) # <class 'int'>
42
print(3.14) # <class 'float'>
3.14
print(1 + 2j) # <class 'complex'>
(1+2j)
print(True) # <class 'bool'>
True
print([1, 2, 3]) # <class 'list'>
[1, 2, 3]
print((1, 2, 3)) # <class 'tuple'>
(1, 2, 3)
print({'red', 'green', 'blue'}) # <class 'set'>
{'red', 'green', 'blue'}
print({'name': 'Alice', 'age': 42}) # <class 'dict'>
{'name': 'Alice', 'age': 42}
print('hello') # <class 'str'>
hello
