Function name in parentheses after function call in Python

Question

I ran into these lines of code in the QPYTHON Android app. They are part of a sample that uses the Bottle module to create a simple Web server that seems to work fine.

app = Bottle()
app.route('/', method='GET')(home)
app.route('/__exit', method=['GET','HEAD'])(__exit)
app.route('/__ping', method=['GET','HEAD'])(__ping)
app.route('/assets/<filepath:path>', method='GET')(server_static)

Now, I know that all the functions in parentheses after the call have already been wrapped with the @route decorator above this. For example:

@route('/__ping')
def __ping():
 return "ok"

But I have no idea what putting things in parentheses after other things does in Python, and after trying a hundred different permutations of "functions in parentheses after functions" I gave up.

I throw myself on the mercy of the Exchange.

• Kilian Foth: It looks like the function route returns another function, and that dynamically selected function is then called with the argument home (for instance). Look up higher-order functions.
• jonrsharpe: "putting things in parentheses after other things" calls the other things - foo(bar, baz) calls foo with the arguments bar and baz, foo(bar)(baz) calls foo with the argument bar and then calls whatever foo returns with the argument baz. In this case, @route is a decorator (i.e. a callable that returns a callable) that takes parameters: stackoverflow.com/questions/5929107/…

Answer 1

This:

app.route('/', method='GET')(home)

... Is the same as this:

func = app.route('/', method='GET')
func(home)

In other words, app.route(...) returns a function, which is then called.

Answer 2

def a():
 def b(arg):
 return arg * 2
 return b
print(a()(5))
c = a()
print(c(5))

Output:
10
10

You are basically providing an argument for the function within the function, a.k.a inner function