When to use dictionary | (merge) vs |= (update) operator

Question

When to use the merge vs update operators on dictionaries.
The following examples, while there are differences in how to call them, their output is the same.

a = {1: 'a', 2: 'b', 3: 'c', 6: 'in both'}
b = {4: 'd', 5: 'e', 6: 'but different'}

Using the update operator

z = a | b    
print(z)

Output: {1: 'a', 2: 'b', 3: 'c', 6: 'but different', 4: 'd', 5: 'e'}

Using The merge operator

a |= b
print(a)

Output: {1: 'a', 2: 'b', 3: 'c', 6: 'but different', 4: 'd', 5: 'e'}

It seems as if the only advantage of the | (merge) is that it doesn't overwrite your old dictionary.
Is there something else that I am missing?
When should I choose to use one over the other?

Answer 1

The |= operator just updates your original dictionary with the result of the union operation. The | operator returns a new dictionary that is the union of the two dicts. Let's say we have two sets

a = {'hello', 'world', 'abc', 'def'}
b = {'abc', 'ghi', 'jkl'}

The operation a |= b is similar to a = a | b in much the same way as a += b is similar to a = a + b for lists.

a = {'hello', 'world', 'abc', 'def'}
al = list(a)
b = {'abc', 'ghi', 'jkl'}
bl = list(b)

print("Before: ", hex(id(a)), a)
a = a | b
print("After: ", hex(id(a)), a)
# Output: 
# Before:  0x1aa0186f128 {'world', 'def', 'abc', 'hello'}
# After:  0x1aa0186f828 {'world', 'ghi', 'hello', 'jkl', 'def', 'abc'}

print("Before: ", hex(id(al)), al)
al = al + bl
print("After: ", hex(id(al)), al)
# Output: 
# Before:  0x1aa0187a248 ['world', 'def', 'abc', 'hello']
# After:  0x1aa0366bdc8 ['world', 'def', 'abc', 'hello', 'jkl', 'abc', 'ghi']

Evidently, a is now a new set at a different location in memory.

a = {'hello', 'world', 'abc', 'def'}
al = list(a)
b = {'abc', 'ghi', 'jkl'}
bl = list(b)

print("Before", hex(id(a)), a)
a |= b
print("After", hex(id(a)), a)
# Output: 
# Before 0x1aa0186f128 {'world', 'def', 'abc', 'hello'}
# After 0x1aa0186f128 {'world', 'ghi', 'hello', 'jkl', 'def', 'abc'}

print("Before", hex(id(al)), al)
al += bl
print("After", hex(id(al)), al)
# Output:
# Before 0x1aa03646888 ['world', 'def', 'abc', 'hello']
# After 0x1aa03646888 ['world', 'def', 'abc', 'hello', 'jkl', 'abc', 'ghi']

In this case, a is still the old set at a same location in memory, but its contents have been updated.