Develop Reference

how can i trigger an action when a variable change it value in this case time?

I need to detect when the minutes of the clock/time change and do something,
This is mi code so far, for the clock but still can figuruate out in python how to detect the value has change and do action after. Any help will be apreciated i come from a c++ backgorund my implementations seems so far not working.
while True:
now = datetime.now()
print(now.strftime("%M), end = " ", flush = true)
time.sleep(1)
currentMin = now.srtftime("%M")

that worked for me:
from datetime import datetime
import time
past_min = None
while True:
#current min
now_min = int(datetime.now().strftime("%M"))
#first iteration
if not past_min:
past_min = now_min
if now_min != past_min:
#call your function here
print("Min change detected")
past_min = now_min
#print the seconds
print(datetime.now().strftime("%S"))
time.sleep(1.5)

I think you can create a class (in the below example Minute) with a property currenMin to store the current minute value. By using #<property>.setter function, when the property <property> is changed, it will trigger the setter function
from datetime import datetime
import time
class Minute(object):
def __init__(self):
self._currenMin = ''
#property
def currentMin(self):
return self._currenMin
#currentMin.setter
def currentMin(self, value):
if value != self._currenMin:
# ACTION CODE BELOW
print('Minute changed')
self._currenMin = value
minute = Minute()
while True:
now = datetime.now()
print(now.strftime("%M"), end=" ", flush = True)
time.sleep(1)
minute.currentMin = now.strftime("%M")

Well, for the general case with simple variables, you can't simply do it. There are two simple options to do something similar:
if you control EVERYTHING that writes it, make them trigger that action
write code that regularly checks it and triggers the action when it changes
use language tools like a custom setter (see #user696969's answer)
The first case needs you to control everything that could modify that value. At that point, you might not even need a variable, and just pass the new value (and you can reverse this by having a variable that is always updated). This is a very common pattern, called Event-driven programming, and heavily used for example in UIs, websites (client-side, see a list of DOM events for example) and game frameworks (see pygame's documentation on events)
The second-case of writing a loop or checking it regularly can also work, however, there are some downsides to it as well. You probably don't want to write an infinite loop waiting for it to change, especially not in a way that also blocks the changing of that variable, and thus dead-locking the entire program as it's preventing something it's waiting for. If you just check it regularly between other, it might be hard to ensure it will be checked regardless of what else is the program doing. You might use multiple threads for it, but that brings it's own set of problems. You also have to store and update the previous value, so you can compare it. This might be slow or memory-consuming if the variable holds too much data.
You can also use language tools with custom setters. This is clean, but can not be used for any variable, just for class attributes, so you still need some control over the rest of the program.
Generally I'd use the event-driven approach or the setter one, depending on the wider context. However, for such simple cases, the checking is also fine. The simplest solution might event be to remove the need for this entirely.

Strange multi-processing trouble

Sorry for the vague title, but since I have no clue what the reason for my problem might be I don't know how to describe it better. Anyway, I have a strange problem in connection with multiprocessing and hope some of you can help me.
I'm currently dealing with convex optimization, especially with parallelizing the tasks. My goal is to utilize as many cores as possible on a multi-core-machine (on which I get access only temporarily).
Therefore, I took a look on the CVXPY's Tutorial page and tried to implement an easy example to get into the topic (scroll down, it's the last example on the page). I shortened the example to the parts which are necessary for my question, so my code looks as follows:
import cvxpy as cp
import numpy
from multiprocessing import Pool
# Assign a value to gamma and find the optimal x.
def get_x(gamma_value):
print("E")
gamma.value = gamma_value
result = prob.solve()
print("F")
return x.value
# Problem data.
n = 15
m = 10
numpy.random.seed(1)
A = numpy.random.randn(n, m)
b = numpy.random.randn(n)
# gamma must be nonnegative due to DCP rules.
gamma = cp.Parameter(nonneg=True)
# Construct the problem.
x = cp.Variable(m)
error = cp.sum_squares(A # x - b)
obj = cp.Minimize(error + gamma*cp.norm(x, 1))
prob = cp.Problem(obj)
# Construct a trade-off curve of ||Ax-b||^2 vs. ||x||_1
sq_penalty = []
l1_penalty = []
x_values = []
print("A")
gamma_vals = numpy.logspace(-4,6, num = 6400)
print("B")
for val in gamma_vals:
gamma.value = val
print("C")
# Parallel computation (set to 1 process here).
pool = Pool(processes = 1)
print("D")
x_values = pool.map(get_x, gamma_vals)
print(x_values[-1])
As you might have observed, I added some prints with capital letters, they served to find out where exactly the issue occurs, so I can refer to them in my problem description;
When I run the code the code processes and the letters "A" to "D" are displayed on the screen, so everything's fine until passing "D". But then the program kind of gets stuck. The CPU load is still high, so there definitely is going on something, but the code never reaches capital "E", which would be after successfully starting
x_values = pool.map(get_x, gamma_vals).
In my eyes it looks a bit like being stuck in an infinite loop.
Therefore, I guess someting with this pool.map function must be fishy. First, I thought that calling this function might be time-consuming and therefore it takes long to process (but that's rubbish, the optimization starts within the get_x function, so no reasoning).
Nevertheless, I tried to run my program on the multi-core-machine (with multiple cores, as well as with only a single core) and - surprise - it successfully passed this line nearly instantaneous and started with the actual optimization problem (and finally solved it).
So my issue is, that I don't know what's going wrong on my computer and how to fix it.
I can't access the machine at any time so - of course - I want to try the code on my computer first before uploading it, which isn't possible if even this easy toy-example doesn't work.
I' be grateful for any ideas/help!
Thank you in advance!
FOOTNOTE: I am using WIN10, the multi-core machine uses Linux

Data-logging from an i2c-connected Atlas Scientific Sensor to a CSV file

I am relatively new to Python, and programming as a whole. I am progressively getting the hang of it, however I have been stumped as of late in regards to one of my latest projects. I have a set of Atlas Scientific EZO circuits w/ their corresponding sensors hooked up to my Raspberry Pi 3. I can run the i2c script fine, and the majority of the code makes sense to me. However, I would like to pull data from the sensors and log it with a time stamp in a CSV file, taking data points in timed intervals. I am not quite sure how to pull the data from the sensor, and put it into a CSV. Making CSVs in Python is fairly simple, as is filling them with data, but I cannot seem to understand how I would make the data that goes into the CSV the same as what is displayed in the terminal when one runs the Poll function. Attached is the i2c sample code from Atlas' website. I have annotated it a bit more so as to help me understand it better.
I have already attempted to make sense of the poll function, but am confused in regards to the self.file_write and self.file_read methods used throughout the code. I do believe they would be of use in this instance but I am generally stumped in terms of implementation. Below you will find a link to the Python script (i2c.py) written by Atlas Scientific
https://github.com/AtlasScientific/Raspberry-Pi-sample-code/blob/master/i2c.py

I'm guessing by "the polling function" you are referring to this section of the code:
# continuous polling command automatically polls the board
elif user_cmd.upper().startswith("POLL"):
delaytime = float(string.split(user_cmd, ',')[1])
# check for polling time being too short, change it to the minimum timeout if too short
if delaytime < AtlasI2C.long_timeout:
print("Polling time is shorter than timeout, setting polling time to %0.2f" % AtlasI2C.long_timeout)
delaytime = AtlasI2C.long_timeout
# get the information of the board you're polling
info = string.split(device.query("I"), ",")[1]
print("Polling %s sensor every %0.2f seconds, press ctrl-c to stop polling" % (info, delaytime))
try:
while True:
print(device.query("R"))
time.sleep(delaytime - AtlasI2C.long_timeout)
except KeyboardInterrupt: # catches the ctrl-c command, which breaks the loop above
print("Continuous polling stopped")
If this is the case then if looks like you can recycle most of this code for your use. You can grab the string you are seeing in your console with device.query("R"), instead of printing it, grab the return value and write it to your CSV.

I think You should add method to AtlasI2C class which will write data to file
Just type under AtlasI2C init() this method:
def update_file(self, new_data):
with open(self.csv_file, 'a') as data_file:
try:
data = "{}\n".format(str(new_data))
data_file.write(data)
except Exception as e:
print(e)
add to AtlasI2C init statement about csv file name:
self.csv_file = <my_filename>.csv # replace my_filename with ur name
and then under line 51 (char_list = list(map(lambda x: chr(ord(x) & ~0x80), list(response[1:]))) add this line:
self.update_file(''.join(char_list))
Hope its gonna help You.
Cheers,
Fenrir

Question with python, music and events, is there a more elegant way of programming this?

Goal: To create a program that will be able to turn off and on lights to music based on events that are triggered from midi notes.
Hello all, I am hoping that this isn't too broad of a question to ask. I am working on a project where I get events from a midi file and turn those events into time. I take a note from the midi file and append it to a list as the time it was placed
Example:
https://imgur.com/swZsrk9
I take all of those and place them into a list. Don't worry about how I do that as that is not my main goal to discuss. I've just substituted the list with a hard-coded one on my example code.
I now have a list of times that I want lights to either turn on or off, now I just need to set an infinite loop with a timer that starts at 0 seconds (with the start of the song) and when the timer == (the next time in the list) it will print out a line. Here is my code:
import socket
import sys
import random
import time
from pygame import mixer
from mido import MidiFile
masterList = [12.37, 14.37, 15.12, 15.62,16.36, 17.61, 18.11, 19.11, 19.61, 20.35,]
mixer.init()
song = mixer.Sound('song.wav')
startTime = time.time()
endTime = startTime + song.get_length()
print(masterList)
print('Starting song.')
song.play()
print('Playing song, timer:',startTime)
while time.time() <= endTime:
#print(round(time.clock(),1),masterList[0])
if round(time.clock(),2) == masterList[0]:
print(round(time.clock(),2),"<-",masterList[0],"------------------")
del masterList[0]
#print('playing...')
time.sleep(.01)
mixer.quit()
Here is a video of it runing:
https://www.youtube.com/watch?v=VW-eNoJH2Wo&feature=youtu.be
Ignore the deprecation warnings
It works but sometimes, due to the nature of programming, the time.clock() does not always == the next item in the list. I knew that this would be a problem going in as you can't rely on how long the code takes to execute. Sometimes it takes a few more milliseconds than usual to complete a while loop so then when you call the time.clock() method it != the next time in the list. Then the list does not remove it's first item and then it will never equal that first item.
I tried going by 10 milliseconds (.1) but it doesn't give me the accuracy I need.
Also, it seems clunky, sometimes the events are delayed a few milliseconds and it makes the effect not as pleasing. As you can see from my video the times of the prints aren't lined up completely where they need to be, even though they are perfectly placed on where they need to be in the midi file.
Question: Is there a more elegant way of tackling this? I seem to keep finding ways of patching it to work better and then it always goes back to the nature of programming, where the cpu always is unreliable. I've been trying to think of different ways of doing this but I can't come up with any. Your help would be much appreciated!!

Since you're deleting the notes as you play them, have you tried using >= instead of ==?
E.g.,:
while time.time() <= endTime:
#print(round(time.clock(),1),masterList[0])
if round(time.clock(),2) >= masterList[0]:
print(round(time.clock(),2),"<-",masterList[0],"------------------")
del masterList[0]
That way the note will play as soon as possible after the specified time, and remove it from the queue. Only way to know actual performance is to test it, but at the very least it won't skip notes.

Return continuous result from a single function call

I have got stuck with a problem.
It goes like this,
A function returns a single result normally. What I want is it to return continuous streams of result for a certain time frame(optional).
Is it feasible for a function to repeatedly return results for a single function call?
While browsing through the net I did come across gevent and threading. Will it work if so any heads up how to solve it?
I just need to call the function carry out the work and return results immediately after every task is completed.

Why you need this is not specified in the question, so it is hard to know what you need, but I will give you a general idea, and code too.
You could return in that way: return var1, var2, var3 (but that's not what you need I think)
You have multiple options: either blocking or non-blocking. Blocking means your code will no longer execute while you are calling the function. Non-blocking means that it will run in parallel. You should also know that you will definitely need to modify the code calling that function.
That's if you want it in a thread (non-blocking):
def your_function(callback):
# This is a function defined inside of it, just for convenience, it can be any function.
def what_it_is_doing(callback):
import time
total = 0
while True:
time.sleep(1)
total += 1
# Here it is a callback function, but if you are using a
# GUI application (not only) for example (wx, Qt, GTK, ...) they usually have
# events/signals, you should be using this system.
callback(time_spent=total)
import thread
thread.start_new_thread(what_it_is_doing, tuple(callback))
# The way you would use it:
def what_I_want_to_do_with_each_bit_of_result(time_spent):
print "Time is:", time_spent
your_function(what_I_want_to_do_with_each_bit_of_result)
# Continue your code normally
The other option (blocking) involves a special kind of functions generators which are technically treated as iterators. So you define it as a function and acts as an iterator. That's an example, using the same dummy function than the other one:
def my_generator():
import time
total = 0
while True:
time.sleep(1)
total += 1
yield total
# And here's how you use it:
# You need it to be in a loop !!
for time_spent in my_generator():
print "Time spent is:", time_spent
# Or, you could use it that way, and call .next() manually:
my_gen = my_generator()
# When you need something from it:
time_spent = my_gen.next()
Note that in the second example, the code would make no sense because it is not really called at 1 second intervals, because there's the other code running each time it yields something or .next is called, and that may take time. But I hope you got the point.
Again, it depends on what you are doing, if the app you are using has an "event" framework or similar you would need to use that, if you need it blocking/non-blocking, if time is important, how your calling code should manipulate the result...

Your gevent and threading are on the right track, because a function does what it is programmed to do, either accepting 1 var at a time or taking a set and returning either a set or a var. The function has to be called to return either result, and the continuous stream of processing is probably taking place already or else you are asking about a loop over a kernel pointer or something similar, which you are not, so ...
So, your calling code which encapsulates your function is important, the function, any function, eg, even a true/false boolean function only executes until it is done with its vars, so there muse be a calling function which listens indefinitely in your case. If it doesn't exist you should write one ;)
Calling code which encapsulates is certainly very important.
Folks aren't going to have enough info to help much, except in the super generic sense that we can tell you that you are or should be within in some framework's event loop, or other code's loop of some form already- and that is what you want to be listening to/ preparing data for.
I like "functional programming's," "map function," for this sort of thing. I think. I can't comment at my rep level or I would restrict my speculation to that. :)
To get a better answer from another person post some example code and reveal your API if possible.