Using reveal_type() to debug types

The last fundamental feature I want to introduce about mypy is the existence of the reveal_type function.

This function doesn't actually exist in Python, in the typing module, or anywhere else. But mypy understands it. reveal_type is used to ask mypy about what it thinks the type of a variable is.

Here's a quick demo:

string = "This is a string"
length = len(string)
reveal_type(length)

So we're able to know that mypy thinks that length is an integer. But if you try to run the code, it will immediately crash, saying reveal_type is not defined.

And this is a feature -- this ensures you don't accidentally leave a reveal_type statement in your code after you're done debugging. Also When you use reveal_type, mypy exits with an error code, saying that the code isn't currently in working shape.

But the important part is, now you can easily know what mypy is thinking about your code, and you can debug your types.

An old friend

Let's use reveal_type to debug an older code example that we saw and fixed:

import json
from itertools import cycle

items_json = """
{
    "items": [
        "item 1", "item 2", "item 3", "item 4", "item 5",
        "item 6", "item 7", "item 8", "item 9", "item 10"
    ]
}
"""

def get_worker_count(): return 4

def run_processes() -> None:
    worker_count = get_worker_count()
    print("Running with " + worker_count + " workers")

    workers = range(worker_count)

    items = json.loads(items_json)
    reveal_type(items)
    for item, worker in zip(items, cycle(workers)):
        print(f"Processing {item} with worker {worker}")


if __name__ == "__main__":
    run_processes()

Using reveal_type, we see a new result: it says the type of items is Any.

We'll talk about Any type and its uses in detail in the next chapter, but you can understand it basically saying "items can be anything". Mypy has no idea what it can be.

With the fixed code, mypy understands the types correctly:

import json
from itertools import cycle
from time import sleep

items_json = """
{
    "items": [
        "item 1", "item 2", "item 3", "item 4", "item 5",
        "item 6", "item 7", "item 8", "item 9", "item 10"
    ]
}
"""

def get_worker_count() -> int: return 4

def run_processes() -> None:
    worker_count = get_worker_count()
    print("Running with " + str(worker_count) + " workers")

    workers = range(worker_count)

    data = json.loads(items_json)
    items = data['items']
    assert isinstance(items, list)
    reveal_type(items)

    for item, worker in zip(items, cycle(workers)):
        reveal_type(item)
        reveal_type(worker)
        print(f"Running {item} with worker {worker}")


if __name__ == "__main__":
    run_processes()

Now mypy is confident that items is a list. But what does it contain? No idea. Could be anything. Similarly, checking the types of item and worker, we get Any and int respectively. Worker is definitely an integer, but item can still be anything. Which is alright: print can print anything out so mypy is okay with the code.

Hopefully that explains a lot more about mypy's workings. Feel free to make changes to the code, and use reveal_type to figure out what mypy thinks.