Store Instance of Class in String

Question

How can I store an instance of a class in a string? I tried eval, but that didn't work and threw SyntaxError. I would like this to work for user-defined classes and built-in classes (int, str, float).

Code:

class TestClass:
    def __init__(self, number):
        self.number = number

i = TestClass(14)
str_i = str(i)
print(eval(str_i)) # SyntaxError
print(eval("jkiji")) # Not Defined
print(eval("14")) # Works!
print(eval("14.11")) # Works!

[Avoid using eval](https://stackoverflow.com/a/1832957/5770501). — Arkistarvh Kltzuonstev, May 06 '21 at 06:46
"How can I store an instance of a class in a string" Use *an actual object serialization format*. The fact that this works with most built-in types doesn't mean you should actually use it this way. Use `pickle`. — juanpa.arrivillaga, May 06 '21 at 07:06

timgeb · Answer 1 · 2021-05-06T06:54:13.527

3

The convention is to return a string with which you could instantiate the same object, if at all reasonable, in the __repr__ method.

class TestClass:
    def __init__(self, number):
        self.number = number
    def __repr__(self):
        return f'{self.__class__.__name__}({self.number})'

Demo:

>>> t = TestClass(14)
>>> t
TestClass(14)
>>> str(t)
'TestClass(14)'
>>> eval(str(t))
TestClass(14)

(Of course, you should not actually use eval to reinstantiate objects in this way.)

edited May 06 '21 at 06:54

answered May 06 '21 at 06:47

timgeb

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yup, this is way that it is supposed to be done, but the user can pass their own classes (creating a very simple data structure using setattr and getattr) – KetZoomer May 06 '21 at 06:59
@KetZoomer this is *definitely not the way it's supposed to be done* – juanpa.arrivillaga May 06 '21 at 18:50
@juanpa.arrivillaga yup, I know :) – KetZoomer May 06 '21 at 19:01

score 2 · Accepted Answer · answered May 06 '21 at 06:51

You can also use the pickle built-in module to convert an object to a byte string and back:

import pickle

class TestClass:
    def __init__(self, number):
        self.number = number

t1 = TestClass(14)
s1 = pickle.dumps(t1)
t2 = pickle.loads(s1)

then

print(t2.number)

will print

Store Instance of Class in String

2 Answers2