[Python] The output changes when changing print function's argument

I have this class that helps me calculate gradient during backpropagation.

import time
visited = set()
class Value:
def __init__(self, data, _children=None, _op=''):
self.data = data
self.grad = 0.0
self._backward = lambda: None
self._prev = set(_children) if _children else set()
self._op = _op

def __add__(self, other):
other = other if isinstance(other, Value) else Value(other)
out = Value(self.data + other.data, (self, other), '+')

def _backward():
self.grad += 1.0 * out.grad
other.grad += 1.0 * out.grad
out._backward = _backward

return out

def __mul__(self, other):
other = other if isinstance(other, Value) else Value(other)
out = Value(self.data * other.data, (self, other), '*')

def _backward():
self.grad += other.data * out.grad
other.grad += self.data * out.grad
out._backward = _backward
return out

def backward(self, output=False):
# if I change 'o' to 'oo' the a.grad changes it's value
# print('o')
# use time.sleep() instead of print
time.sleep(0.1)
if output:
self.grad = 1.0
self._backward()
for child in self._prev:
if child not in visited:
visited.add(child)
child.backward()

def __repr__(self):
return f"Value(data={self.data}, grad={self.grad})"

a = Value(2.0)
b = Value(3.0)
c = a + b
d = a * b
e = c + d
g = c * d
f = e + g

f.backward(True)
print(f"a grad: {a.grad}")


In function backward() if I change the print's argument the output for a.grad will change too.

For print('o') output will be :

o
o
o
o
o
o
o
a grad: 21.0


For print('oo') output will be:

oo
oo
oo
oo
oo
oo
oo
a grad: 4.0


So a.grad change from 21.0 -> 4.0. Why is that?

Update

It seems that it's easier to reproduce using time.sleep(). I have updated code from print() to -> time.sleep(0.1)

With time.sleep(0.1):

a.grad = 21.0
​

Without time.sleep(0.1):

a.grad = 4.0
​

P.S. I know that backward() doesn't implement backpropagation correctly.

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