How do I reverse str(class) in Python?

Question

I need to define a new instance of a class from an informal string representation of the same type of class. What is a clean way to do this in Python?

program1.py:

fileHandle = open("filename.txt", "wb")
instance = className()
instance.a = 2
instance.b = 3.456
fileHandle.write(str(instance))

filename.txt (after running program1.py):

<className a=2, b=3.456>

program2.py:

instance = className()
with open("filename.txt", "r") as fileHandle:
    for fileLine in fileHandle:
        ##### How do I grab the contents of the file line and get them in variables? #####
        (instance.a, instance.b) = magicFunction(fileLine)
        # I seem to have forgotten the contents of magicFunction(). Can someone remind me?

Answer 1

In general, the python str function is designed for printing things in a human-readable way, not computer-readable as you want to use. If you control program1.py , the pickle module could fit your needs.

Program1.py:
    import pickle
    [rest of code unrelated to printing]
    pickle.dump(instance,fileHandle)
Program2.py:
    instance = pickle.load(fileHandle)

Answer 2

The __repr__ magic method is intended for this purpose. It should return the string that, when evaluated in a Python interpreter or source code file, would generate the same object.

class className:
    def __init__(self, a=0, b=0):
        self.a = a
        self.b = b
    def __repr__(self):
        return 'className(a={}, b={})'.format(self.a, self.b)

instance = className()
instance.a = 2
instance.b = 3.456
print(repr(instance))

with open("filename.txt", "w") as fileHandle:
    fileHandle.write(repr(instance))

with open("filename.txt", "r") as fileHandle:
    my_instance = eval(fileHandle.read())
    print(repr(my_instance))

However, if all you really want to do is save the object to a file and then read it back in later, you could use a module like json or pickle .