Python help with GPIO

Question

Okay, I guess it is time to ask for help. I have learned how to breadboard up a set of traffic lights, a button and a buzzer. I can get them all to work with simple python programs. Now I want to interrupt the normal light sequence when the button is pushed. I can even get a detect event to work. What I do not seem to be able to do is condition the "if" or "while" loop for the lights on the event.

I have a callback and it seems to work, so I can print the status of the "button is pressed" but I cannot pass it as "true" to the loop. I suspect it may be that when defining "btnpushed" it is local to only the callback loop?

i can self learn, but most of the tutorials I see are very single topic context, like what is a while loop or what is an if loop. Any pointers to a good example of using these functions would be great!

i hope this is clear.

Roger Ayotte

colporteur · Answer

Any chance of getting a peek at your code? Coding problems through description is like some describing what they see in an abstract painting. I don't have the level of python code experience of some members but I do have a pi I can experiment with to analyze errors.

fmilburn · Answer

Hi Roger, A global variable can be defined in Python but it is not clear to me that is the issue. Try making the variable to be used as a flag global and if that doesn't work post your Python code with as much of the superfluous removed as you can that still demonstrates the issue. It will be easier to read if the Insert tool with syntax is used: Frank

genebren · Answer

Hi Roger, Frank is right, if possible post code (minimal to describe the problem). I am not a Python person, but a quick search ("python callback return value") did return a possible solution, see here: https://stackoverflow.com/questions/19585935/returning-values-from-callbacks-in-python Best of luck! Gene

clem57 · Answer

Let me paraphrase a bit. You have something going in on a loop. You have a callback setup on a button push. And you wish to affect the looping code. Right? Here is the problem. When the callback runs. you do not have access to the stuff running! Look at the routine or function that setup the callback. I will bet you can pass a parameter which you set as TRUE or FALSE initially. Within the callback, change it to the NOT condition.The looping code can test the value and know when the callback changed it. Clem

shabaz · Answer

As Frank mentions, the global keyword may be missing (but hard to know without seeing the code).

From what little I know, Python behavior is backward compared to some older languages, in that the global declaration has to be within your function (i.e. the variable is declared as normal outside, but is only considered global within each function if the function declared it as a global, otherwise a local instance gets used by the function). I could be wrong, so please do check some Python user guide.

It seems modern languages have moved in this way, that you have to be a lot more descriptive explicitly about your variable scopes etc. in this more fine-grained way where for every function you wish to use the variable, you have to explicitly specify its scope as opposed to doing it one-time at the top of the code.

Some modern languages go further, and you can express even more about variable values and the compiler can then catch unusual stuff like a variable being used differently in one function compared to others. This maybe feels more unnatural to users of older languages (it feels unnatural to me, but I can see why it's being done). Maybe newcomers will have the advantage that they find it immediately normal, if they have not known that older languages like C let you pretty much do anything (with all the benefits and disadvantages of that).

Incidentally, there was a recent question about a similar thing (it comes up a lot! usually Arduino-code-based, but also Pi and Python too), about trying to come up with a structure for doing stuff in parallel and communicating between pieces of code when an external event occurs, in Python. It is a popular scenario, since most real-time applications need to respond to interrupt-like events. I can't recall it, but it was definitely within the past few weeks, and some examples were given of different ways of doing it for Python. The response comments from that question ought to be turned into a blog post really, since it will just get lost in the noise otherwise.

fmilburn · Answer

Incidentally, there was a recent question about a similar thing... found it :-) https://www.element14.com/community/message/286769/l/re-writing-threads-into-code-for-4-channel-relay-on-pi#286769

fmilburn · Answer

Do you intend for line 17 to have == instead of =

easyejl · Answer

well, the single = is what is correct there, but then after that you still have to deal with the issue that the callback comes in on a separate thread from the main thread, and its only a 1 in the thread that is the callback, not in main.

shabaz · Answer

rcayot wrote: I happened to be trying that, it works when only = instead of == I added a print statement after btnpushed = 0 and at the start of the program it prints 0, and when I press the button, it prints 1 but on every case the lights keep cycling. Roger That's because every variable you're modifying in that def block is _local_. You need to define it as global.

genebren · Answer

Roger, In your code you are confusing the meaning of "=" and "==". In the statement inside of the callback (cb1) you have a statement "btnpushed == 1". This should be "btnpushed = 1". The first is a test, comparing btnpushed to 1, where the other is an assignment setting btnpushed to 1. Good luck! Gene