Python update a key in dict if it doesn't exist

Question

I want to insert a key-value pair into dict if key not in dict.keys(). Basically I could do it with:

if key not in d.keys():
    d[key] = value

Answer 1

According to the above answers setdefault() method worked for me.

old_attr_name = mydict.setdefault(key, attr_name)
if attr_name != old_attr_name:
    raise RuntimeError(f"Key '{key}' duplication: "
                       f"'{old_attr_name}' and '{attr_name}'.")

Though this solution is not generic. Just suited me in this certain case. The exact solution would be checking for the key first (as was already advised), but with setdefault() we avoid one extra lookup on the dictionary, that is, though small, but still a performance gain.