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From here, it seems like I am wrong, to be updated asap.

Is this as simple as it can be ? Not quite ? The check for d.get(my_key) is actually to distinguish "test3" from the other values. It could be easier to just do the same thing for all values and then override the "test3" value.

def my_function(my_string, my_key):
    d = {'test' + str(i): {} for i in range(3)}
 
    if my_string:
        d[my_key] = {my_string: None}
    else:
        d[my_key] = {'NULL': None}
    d['test3'] = {'a_key': 'a_value'}
    return d

Now, you have 2 branches looking pretty similar, let's factorize this out.

def my_function(my_string, my_key):
    d = {'test' + str(i): {} for i in range(3)}
    d[my_key] = {my_string if my_string else 'NULL': None}
    d['test3'] = {'a_key': 'a_value'}
    return d

Is this as good as it can be ? I think so but I might be wrong.

Is this as simple as it can be ? Not quite ? The check for d.get(my_key) is actually to distinguish "test3" from the other values. It could be easier to just do the obvious thing instead of trying to retrieve stuff from the dict :

def my_function(my_string, my_key):
    d = {'test' + str(i): {} for i in range(3)}
    d['test3'] = {'a_key': 'a_value'}
    if my_string:
        d[my_key] = {my_string: None}
    elif my_key != 'test3': 
        d[my_key] = {'NULL': None}
    return d

# my_string not blank, my_key is new
assert my_function('hello', 'test4') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {}, 'test1': {}, 'test4': {'hello': None}}

# my_string not blank, my_key exists
assert my_function('hello', 'test0') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {'hello': None}, 'test1': {}}

# my_string blank, my_key exists and its value is not empty
assert my_function('', 'test3') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {}, 'test1': {}}

# my_string blank, my_key exists and its value is empty
assert my_function('', 'test0') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {'NULL': None}, 'test1': {}}

# my_string blank, my_key is new
assert my_function('', 'test4') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {}, 'test1': {}, 'test4': {'NULL': None}}

# my_string not blank, my_key is new
assert my_function('hello', 'test4') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {}, 'test1': {}, 'test4': {'hello': None}}

# my_string not blank, my_key exists
assert my_function('hello', 'test0') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {'hello': None}, 'test1': {}}

# my_string blank, my_key exists and its value is not empty
assert my_function('', 'test3') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {}, 'test1': {}}

# my_string blank, my_key exists and its value is empty
assert my_function('', 'test0') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {'NULL': None}, 'test1': {}}

# my_string blank, my_key is new
assert my_function('', 'test4') == {'test2': {}, 'test3': {'a_key': 'a_value'}, 'test0': {}, 'test1': {}, 'test4': {'NULL': None}} 

# New test from Sjoerd Job Postmus's comment
assert my_function('something not empty', 'test3') == {'test1': {}, 'test0': {}, 'test3': {'something not empty': None}, 'test2': {}}

From here, it seems like I am wrong, to be updated asap.

Is this as good as it can be. I think so but I might be wrong.

Is this as good as it can be ? I think so but I might be wrong.