CODEWITHSUNDEEP
pythonscope
Carl Houtman
Carl Houtman

Reputation: 165

Why does my function only see some of my globals?

Ok, I'm a C, VisualBasic, and Fortran programmer (yes we do still exist). I am missing a paradigm piece of Python 3.2. In the code below, why doesn't my function start_DAQ see all my globals? The function seems to make its own local variables for do_DAQ, data_index, and start_time, but not store_button. I've read the response to several similar questions and still don't get it. I imagine I could use a global statement, but was told that that was bad practice.

#-----------------------------------------------------------------------
#      define my globals
#-----------------------------------------------------------------------

#make an instance of my DAQ object and global start_time
myDaq = DAQ_object.DAQInput(1000,b"Dev2/ai0")
start_time = time.time()

#build data stuctures and initialize flags
time_data = numpy.zeros((10000,),dtype=numpy.float64)
volt_data = numpy.zeros((10000,),dtype=numpy.float64)
data_queue = queue.Queue()
data_index = 0
do_DAQ = False

#-----------------------------------------------------------------------
#      define the functions associated with the buttons
#-----------------------------------------------------------------------

def start_DAQ():
    do_DAQ = True
    data_index=0
    start_time = time.time()
    store_button.config(state = tk.DISABLED)

below this is some code for building a GUI

#make my root for TK
root = tk.Tk()

#make my widgets
fr = tk.Frame()
time_label = tk.Label(root, text="not started")
volt_label = tk.Label(root, text="{:0.4f} volts".format(0))
store_button = tk.Button(root, text="store Data", command=store_DAQ, state = tk.DISABLED)
start_button = tk.Button(root, text="start DAQ", command=start_DAQ)
stop_button = tk.Button(root, text="stop DAQ", command=stop_DAQ)
exit_button = tk.Button(root, text="Exit", command=exit_DAQ)

Upvotes: 2

Views: 109

Answers (1)

StoryTeller - Unslander Monica
StoryTeller - Unslander Monica

Reputation: 170084

Add the following to your function:

def start_DAQ():
    global do_DAQ 
    global data_index
    global start_time

    do_DAQ = True
    data_index=0
    start_time = time.time()
    store_button.config(state = tk.DISABLED)

Python implements name hiding in local scope upon write. When you try to read a global(module scoped variable), the interpreter looks for the variable name in increasingly no-local scopes. When you attempt to write, a new variable is created in local scope, and it hides the global.
The statement we added, tells the interpreter to look for the name in global scope upon write. There is also a nonlocal statement (in python 3.*, not sure about 2.7) which has the interpreter write to the variable in the closest nolocal scope, rather than just in module scope.

Upvotes: 7

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