Python help with GPIO

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.

Here is an example of simple callback in Python https://en.wikipedia.org/wiki/Callback_%28computer_programming%29 .The harder one is non blocking or asynchronous. Why? Because you must be careful sharing data. To avoid a race condition, only update a variable in the callback and no where else while the callback is active, Global variables can be used if correctly defined in both caller and callback routines. But it is safer to pass a parameter into the callback!

Clem

Here is an example of simple callback in Python https://en.wikipedia.org/wiki/Callback_%28computer_programming%29 .The harder one is non blocking or asynchronous. Why? Because you must be careful sharing data. To avoid a race condition, only update a variable in the callback and no where else while the callback is active, Global variables can be used if correctly defined in both caller and callback routines. But it is safer to pass a parameter into the callback!

Clem

That's a pretty bad article. Isn't it just referring to functions as arguments (aka function pointers if we're talking about C) in the examples? Callbacks are asynchronous, and therefore differ in that when they get executed, they get executed from a different thread.

Using a function as an argument, and accepting the return value doesn't sound like a callback at all. Maybe it is, in which case I've been using the wrong terminology for years (likely - I'm not a comp-sci).

But the general point is valid, that there can be issues with global variables over other methods of communication. It's a simple method, and can cause wrong results if written from multiple threads.

A callback can be blocking or not. The blocking kind are a bit lame in my book. The wiki is not well written, but I only posted for the simple example in Python. Not sure of the rest of the stuff! The take away is use parameter passing because it can be handled in a safe manner more than global variables where YOU are responsible for the safety. Too many time I have seen careless coding example(s) which turn into production!

Clem

P.S. Here is a better written article describing callbacks.

In computer programming, a callback is a piece of executable code that is passed as an argument to other code, which is expected to call back (execute) the argument at some convenient time. The invocation may be immediate as in a synchronous callback or it might happen at later time, as in an asynchronous callback.

That article is quoting from Wikipedia. In any event, the Wikipedia examples for both C and Python are just examples of passing functions as arguments, since it is all executing in a single thread. A real callback would need to execute in a different thread if it was to be able to ever work asynchronously. (I guess! I'm not an expert..).