Anonymous function in place of function ref in python

Question

in threading.timer you pass the name of a function (example below). Is it possible to replace this with the function body so I dont have to bother defining another trivial function

def PulseLed(self):
    GPIO.output(26, GPIO.HIGH)
    self.ledTimer = threading.Timer(1, self.TurnOffLed)
    self.ledTimer.start()

def TurnOffLed(self):
    GPIO.output(26, GPIO.LOW)

You are not passing name in this example, you are passing a method object. Why not try to simply replace it with anonymous function? — Tadeck
– Tadeck, Commented Sep 26, 2013 at 0:01

Tim Peters · Accepted Answer · 2013-09-25 23:58:38Z

6

Python's syntax does not allow including statements in argument lists. Often you can use a lambda, though, like:

threading.Timer(1, lambda: GPID.output(26, GPIO.LOW))

Overused, this can lead to unreadable code. You know it's "overused" if you start to stuff multiple x if b else y clauses into your lambda ;-)

answered Sep 25, 2013 at 23:58

Tim Peters

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prgao · Accepted Answer · 2013-09-25 23:56:28Z

2

self.ledTimer = threading.Timer(1, lambda: GPIO.output(26, GPIO.LOW))

answered Sep 25, 2013 at 23:56

prgao

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Veedrac · Accepted Answer · 2013-09-25 23:58:28Z

2

I'm always partial to partial:

from functools import partial

def PulseLed(self):
    GPIO.output(26, GPIO.HIGH)
    self.ledTimer = threading.Timer(1, partial(GPIO.output, 26, GPIO.LOW))
    self.ledTimer.start()

answered Sep 25, 2013 at 23:58

Veedrac

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2 Comments

Tadeck Over a year ago

I think using functools.partial is an overkill (simple lambda would do), but it still works. Upvoting.

abarnert Over a year ago

When all you want to do is bind some static positional arguments, it's hard to see how partial is overkill, or underkill. When you start doing convoluted things with it, or trying to combine it with a compose function, that's when you realize you're not actually writing Python anymore… (Well, that's when I realize; maybe some people realize it faster.)

user2246674 · Accepted Answer · 2013-09-26 00:09:30Z

2

I like the "trivial function" in many cases; however, I would write it similar to following. I find that using local functions - just as with using local variables - can add "self documentation" to code.

def PulseLed(self):
    def tunOffLed():
        GPIO.output(26, GPIO.LOW)
    GPIO.output(26, GPIO.HIGH)
    self.ledTimer = threading.Timer(1, turnOffLed)
    self.ledTimer.start()

(Perhaps even put the other GPIO call in a turnOnLed function.)

answered Sep 26, 2013 at 0:09

user2246674

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2 Comments

abarnert Over a year ago

If we had PEP 403 you could even make it statement-local instead of function-local. Which really isn't any more readable in this case, or in most cases, which is why Nick Coghlan deferred the PEP, but some day someone will find a killer use case for it…

user2246674 Over a year ago

@abarnert I can't say I like the syntax proposed, but it's a nice read (thanks for the link!). Some languages allow similar constructs already, showing that there is merit with some code/cases (question is then if it fits the Python Way well). And, for some reason, such a construct always appears whenever I fiddle around with syntax design for never-materializing pet languages ..

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