Find previous value of a variable in Python

Question

This might be a really strange question but nevertheless,

Consider a variable called a. Let us now assign it a value as follows:

a = 1

Let us now change the value of a now.

a = 3

Is there any way in Python to know the previous value of a variable without storing it in another variable.Does python internally maintain a sort of a ledger of all the values of a variable during its lifetime that can be accessed ?

Answer 1

Answer: actually, we CAN

But not in general case.

You need some magick for this.

And magick is called "custom namespaces".

Whole idea is from Armin Ronacher presentation 5 years of Bad Ideas.

Magick: custom namespace with history of values

Let's create custom namespace that saves history of values.

For demonstration purposes let's change rule for __del__ - instead of deleting values we will insert None.

from collections import MutableMapping
class HistoryNamespace(MutableMapping):
    def __init__(self):
        self.ns = {}        
    def __getitem__(self, key):
        return self.ns[key][-1]  # Rule 1. We return last value in history 
    def __delitem__(self, key):
        self.ns[key].append(None) # Rule 4. Instead of delete we will insert None in history
    def __setitem__(self, key, value): # Rule 3. Instead of update we insert value in history
        if key in self.ns:
            self.ns[key].append(value)            
        else:
            self.ns[key] = list([value,]) # Rule 2. Instead of insert we create history list
    def __len__(self):
         return len(self.ns)
    def __iter__(self):
         return iter(self.ns)    

history_locals = HistoryNamespace()        
exec('''
foo=2
foo=3
del foo  
foo=4
print(foo)
''', {}, history_locals)
print("History of foo:", history_locals.ns['foo'])

Rejoice!

Custom namespaces is very powerful technique but almost never used.

The fact I find somewhat puzzling.