I posted a Python tidbit about how for loops can assign to other things than simple variables, and many people were surprised or even concerned:

params = {
    "query": QUERY,
    "page_size": 100,
}

# Get page=0, page=1, page=2, ...
for params["page"] in itertools.count():
    data = requests.get(SEARCH_URL, params).json()
    if not data["results"]:
        break
    ...

This code makes successive GET requests to a URL, with a params dict as the data payload. Each request uses the same data, except the “page” item is 0, then 1, 2, and so on. It has the same effect as if we had written it:

for page_num in itertools.count():
    params["page"] = page_num
    data = requests.get(SEARCH_URL, params).json()

One reply asked if there was a new params dict in each iteration. No, loops in Python do not create a scope, and never make new variables. The loop target is assigned to exactly as if it were an assignment statement.

As a Python Discord helper once described it,

While loops are “if” on repeat. For loops are assignment on repeat.

A loop like for <ANYTHING> in <ITER>: will take successive values from <ITER> and do an assignment exactly as this statement would: <ANYTHING> = <VAL>. If the assignment statement is ok, then the for loop is ok.

We’re used to seeing for loops that do more than a simple assignment:

for i, thing in enumerate(things):
    ...

for x, y, z in zip(xs, ys, zs):
    ...

These work because Python can assign to a number of variables at once:

i, thing = 0, "hello"
x, y, z = 1, 2, 3

Assigning to a dict key (or an attribute, or a property setter, and so on) in a for loop is an example of Python having a few independent mechanisms that combine in uniform ways. We aren’t used to seeing exotic combinations, but you can reason through how they would behave, and you would be right.

You can assign to a dict key in an assignment statement, so you can assign to it in a for loop. You might decide it’s too unusual to use, but it is possible and it works.