Re: [Tutor] counting function calls
On 13/04/17 17:10, marcus lütolf wrote:
> Dear experts, Mats
> I have found the solution, I put the counting variable at the wrong place:
I don;t think so, what you have done now is count the times
through the loop, but thats not (always) the same as the
number of times the function gets called, which is what
you said you wanted to count..
>> #!/usr/bin/python3
>> import sys, time
>> import RPi.GPIO as gpio
>>
>> gpio.setmode(gpio.BOARD)
>> gpio.setup(23, gpio.IN)
>> count = 0
>> def mein_callback(pin):
>> count += 1
This line will still give you an error.
>> print('PIR 1 aktiviert', count)
>> return
>>
>> try:
>count = 0
>> gpio.add_event_detect(23, gpio.RISING, callback = mein_callback)
>> while True:
>> time.sleep(2)
>count += 1
This just counts how many times your while loop goes
round - once every 2 seconds. It says nothing about
how often the callback gets executed.
To do that you need to add the line
global count
to your callback function.
But then both the loop and function will increment
the global count variable so you need to remove
(or rename) the one in the loop.
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos
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Re: [Tutor] counting function calls
Dear experts, Mats
I have found the solution, I put the counting variable at the wrong place:
> #!/usr/bin/python3
> import sys, time
> import RPi.GPIO as gpio
>
> gpio.setmode(gpio.BOARD)
> gpio.setup(23, gpio.IN)
> count = 0
> def mein_callback(pin):
> count += 1
> print('PIR 1 aktiviert', count)
> return
>
> try:
count = 0
> gpio.add_event_detect(23, gpio.RISING, callback = mein_callback)
> while True:
> time.sleep(2)
count += 1
> except KeyboardInterrupt:
> print('PIR deaktiviert')
Marcus.
-Ursprüngliche Nachricht-
Von: Mats Wichmann [mailto:m...@wichmann.us]
Gesendet: Montag, 10. April 2017 15:15
An: marcus lütolf ; tutor@python.org
Betreff: Re: [Tutor] counting function calls
On 04/10/2017 01:55 AM, marcus lütolf wrote:
> Dear experts,
> I have written the following code for motion detection with a PIR
> sensor with a function and I need to count how many times the funtion
> is called, but I get a traceback:
>
> #!/usr/bin/python3
> import sys, time
> import RPi.GPIO as gpio
>
> gpio.setmode(gpio.BOARD)
> gpio.setup(23, gpio.IN)
> count = 0
> def mein_callback(pin):
> count += 1
> print('PIR 1 aktiviert', count)
> return
>
> try:
> gpio.add_event_detect(23, gpio.RISING, callback = mein_callback)
> while True:
> time.sleep(2)
> except KeyboardInterrupt:
> print('PIR deaktiviert')
>
> PIR 1 aktiviert
> Traceback (most recent call last):
> File "./PIRex.py", line 9, in mein_callback
> count += 1
> UnboundLocalError: local variable 'count' referenced before assignment
> ^CPIR deaktiviert
>
> Tanks for help, marcus.
Yes, what Python does here may be surprising at first: if you only
read-access a global variable in a local (function in this case) scope, it
gives you the value just fine. If you however try to save something to a
global variable what happens is it creates a local variable,
*unless* you have previously informed Python you mean the global one, by
using the global statement as Alan listed. The specific error you see is
because in order to increment 'count' (which Python has already figured out
has to be local because it will be assigned to) you have to read the
existing value first, but there is no existing value in the local scope.
The Python programming FAQ has a short explanation of why this might be so:
https://docs.python.org/2/faq/programming.html#what-are-the-rules-for-local-
and-global-variables-in-python
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Re: [Tutor] counting function calls
On 04/10/2017 01:55 AM, marcus lütolf wrote:
> Dear experts,
> I have written the following code for motion detection with a PIR sensor
> with a function and
> I need to count how many times the funtion is called, but I get a traceback:
>
> #!/usr/bin/python3
> import sys, time
> import RPi.GPIO as gpio
>
> gpio.setmode(gpio.BOARD)
> gpio.setup(23, gpio.IN)
> count = 0
> def mein_callback(pin):
> count += 1
> print('PIR 1 aktiviert', count)
> return
>
> try:
> gpio.add_event_detect(23, gpio.RISING, callback = mein_callback)
> while True:
> time.sleep(2)
> except KeyboardInterrupt:
> print('PIR deaktiviert')
>
> PIR 1 aktiviert
> Traceback (most recent call last):
> File "./PIRex.py", line 9, in mein_callback
> count += 1
> UnboundLocalError: local variable 'count' referenced before assignment
> ^CPIR deaktiviert
>
> Tanks for help, marcus.
Yes, what Python does here may be surprising at first: if you only
read-access a global variable in a local (function in this case) scope,
it gives you the value just fine. If you however try to save something
to a global variable what happens is it creates a local variable,
*unless* you have previously informed Python you mean the global one, by
using the global statement as Alan listed. The specific error you see is
because in order to increment 'count' (which Python has already figured
out has to be local because it will be assigned to) you have to read the
existing value first, but there is no existing value in the local scope.
The Python programming FAQ has a short explanation of why this might be so:
https://docs.python.org/2/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python
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Re: [Tutor] counting function calls
On 10/04/17 08:55, marcus lütolf wrote:
> Dear experts,
> I have written the following code for motion detection with a PIR sensor
> with a function and
> I need to count how many times the funtion is called, but I get a traceback:
>
> #!/usr/bin/python3
> import sys, time
> import RPi.GPIO as gpio
>
> gpio.setmode(gpio.BOARD)
> gpio.setup(23, gpio.IN)
> count = 0
> def mein_callback(pin):
> count += 1
> print('PIR 1 aktiviert', count)
> return
> UnboundLocalError: local variable 'count' referenced before assignment
> ^CPIR deaktiviert
You are trying to modify a variable defined in the module or
global scope. To do that you must tell Python that it is
the global variable you mean. You do that by adding
global count
at the top of your function:
def mein_callback(pin):
global count # use the global variable
count += 1
print('PIR 1 aktiviert', count)
return
You don;t need the return since Python returns None automatically
at the end of a function. But it doesn't do any harm either...
HTH
--
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos
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