Unintended recursive loop with repr method

Question

I want to display a list of all accessible attribute of an instance, so i try this

class Test:
    a=2     #initializing some attribute
    b=1     #initializing some attribute
    def some_method(self):
        attr=[]                 
        for k in [i for i in dir(self) if not i.startswith('__')]:
            attr.append(f'{k}={getattr(self,k)}')      #to collect all attributes and their values pair
        return attr
    def __repr__(self):                                #because i want to overload print behavior
        return str(self.some_method())
some_instance=Test()
print(some_instance)

And it returned an error:

RecursionError: maximum recursion depth exceeded while calling a Python object

Why did that happen ? And how to fix that ?

Well if there is a cycle in your object (for example the list contains a reference to `self`, or one of the elements somehow aims to print `self`), then you get in a recursive loop. — Willem Van Onsem, Jul 27 '19 at 11:15
@WillemVanOnsem `a`,`b`,`some_method` are everything i have in the instance, which of them cause this problem ? — Tuấn Đoàn, Jul 27 '19 at 11:20
@TuanDoan: usually `dir` has more than only these attributes. For example `some_method`, etc. The `repr(..)` of `Test().some_method` is ` — Willem Van Onsem, Jul 27 '19 at 11:23

score 1 · Accepted Answer · answered Jul 27 '19 at 11:55

As pointed out in the comments, repr(Test().some_method) calls repr of the object where it lives, yielding in a never ending son-father repr method call stack. If you are not really interested in showing callable members you can filter them out so that they never get asked to be part of a string:

class Test:
    a=2     #initializing some attribute
    b=1     #initializing some attribute
    def some_method(self):
        attr=[]                 
        for k in dir(self):
            val = getattr(self, k)
            if not k.startswith('__') and not callable(val):
                attr.append(f'{k}={val}')      #to collect all attributes and their values pair
        return attr
    def __repr__(self):                                #because i want to overload print behavior
        return str(self.some_method())
some_instance=Test()

Then print(some_instance) shows

['a=2', 'b=1']

delta delta · Answer 2 · 2020-06-26T11:32:50.457

1

You can use __str__ instead of __repr__ to overload printing. In case you must use __repr__ then use the isinstance() to compare the type of attribute values against types.MethodType in the standard library.

edited Jun 26 '20 at 11:32

answered Jun 25 '20 at 15:32

delta delta

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