Why does a function raise UnboundLocalError for a name that exists globally?

The assignment is the key. Python treats a name bound anywhere in a function as local throughout that function, unless it is declared global or nonlocal. So count += 1 tries to read the function’s local count before that local has a value; the global count does not supply it. That read-before-local-binding situation raises UnboundLocalError. [S1]

For a minimal reproduction, the example you posted is enough: the global value exists, but the function’s own binding decision makes the attempted read local. The error is about scope, not whether the module-level variable was created.

Temporarily remove the assignment and read the name:

count = 10

def show_count():
    return count

print(show_count())  # 10

With no binding of count in show_count, the function can use the global name. Add an assignment to that same name anywhere in the function, even below a read, and Python treats it as local throughout the function unless you declare otherwise. [S1]

For example, this still fails on the first line of the function body:

def show_then_change():
    print(count)
    count = 11

The later assignment matters when Python determines the name’s scope; it does not make the earlier read use the global.

Use a different local name, or pass the starting value in and return the result. These approaches make the data flow explicit and avoid rebinding the global:

count = 10

def next_count(current):
    updated = current + 1
    return updated

count = next_count(count)

Here current is the function’s input and updated is its local result. The caller decides whether to store that result back in count. If the calculation is only for display or another operation, keep the returned value in a different caller-side name instead:

new_count = next_count(count)

Use global only when rebinding the module-level name from inside the function is intentional. For example:

count = 10

def add_one():
    global count
    count += 1
    return count

The declaration directs uses of count in that function to the module-level namespace. [S1] If you mean a name in an enclosing function instead, nonlocal refers to a previously bound name in the nearest enclosing function scope; it does not refer to a module global. [S1]

Before adding either declaration, ask whether the function should instead take an input and return a result. That option usually makes it clearer where the value comes from and where the change is used. The cited language reference explains the scope rule, but it does not prescribe which design is best for your particular program.