Python Object Lookup Process

Whenever a variable, function, or module is invoked in Python, there is a hierarchy that determines where it looks for these objects.

• First, Python looks inside the locals() array, which has entries for all local variables. Python works hard to make local variable lookups fast, and this is the only part of the chain that doesn’t require a dictionary lookup.
  • __local variables__ do not need a dictionary lookup to be found; they are stored in a very slim array that has very fast lookup times. Because of this, finding the function is quite fast!
• If it doesn’t exist there, the globals() dictionary is searched.
• Finally, if the object isn’t found there, the __builtin__ object is searched.

It is important to note that while locals() and globals() are explicitly dictionaries and __builtin__ is technically a module object, when searching __builtin__ for a given property, we are just doing a dictionary lookup inside its locals() map (this is the case for all module objects and class objects!).

As a result of the above:

• It is recommended to import specific objects (variables/functions/classess…) from a module instead of just importing the module and use the module name to refer to the objects it contains because this will slow down the run-time.
• The performance of the dictionary lookup is largely dependent of the hash function. Therefore, the slower the hash function, the slower the lookup and may not have O(1) lookup anymore.