IPython auto completion is calling __getattr__

Question

thanks for taking the time to read this. Hope everything is alright with you on these strange times.

I'm implementing a class and started looking how to provide auto-completion on its attributes. From my research online i reached the conclusion that ipython completions come from the __dir__ method.

__getattr__ is normally called when you access an attribute that doesn't exist. In my project if that happens, an operation that takes a while runs. Why is ipython trying to access the attributes instead of just displaying what __dir__ returned?

In cell 2 i hit tab after the dot to ask for completions.

Answer 1

I think the issue is that you need an instance of your class. Those methods are instance methods.

I added logging for easier debugging. I am getting output in the ipython console when I example.something or example.<tab>.

• Python version 3.8.2
• IPython version 7.14.0

This is my observation: On <tab> __dir__ is called and the returned collection of items are displayed in IPython console. If the item chosen after <tab> is not an attribute of the object, then __getattr__ is called in an attempt to look it up.

import logging

logger = logging.getLogger()
logger.setLevel(logging.INFO)


class Example:
    def __init__(self):
        logging.info("init")
        self._attrs = ("foo", "bar", "baz")
        for attr in self._attrs:
            setattr(self, attr, attr)

    def __getattr__(self, attr):
        logging.info(f"__getattr__ called: {attr}")

    def __dir__(self):
        logging.info("__dir__ called")
        return ("extra", *self._attrs)


# Create an instance of Example.
# The instance methods can then be called on the instance.
example = Example()

if __name__ == "__main__":
    logging.info(example)

Answer 2

So I figured out a workaround for this. Provided that you know the attribute name that takes a long time, only do the custom getattr code when that attribute name is passed, else just raise an attribute error. This worked for me.