Watching a variable in python?

Question

In C++, I could imagine constructing with a reference to a counter ( see below ), and then a function would just dereference the address to get a value. Is something similar possible in python?

Something like:

import time
class Count_Watcher:

    def __init__( self, to_watch ):

        self.to_watch = to_watch
        sleep_watch()

    def sleep_watch( self ):

        time.sleep( 5 )
        print( self.to_watch )

line_counter = 0
separate_thread_printer = Count_Watcher( (?some operator) line_counter )

for line in some_file:
    line_counter += 1

and the "current" value of line_counter ( as in, current to the for loop ) is printed every five seconds

Nope, not possible in Python. Your best bet is to make the counter an attribute of the Count_Watcher object. — Cory Kramer
– Cory Kramer, Commented Jun 22, 2015 at 17:55
You would have to pass the variable's name as a string ('line_counter'), and then you'd have to know what namespace to look it up in (e.g., the global namespace in this case). — BrenBarn
– BrenBarn, Commented Jun 22, 2015 at 17:56
@CoryKramer But if I make it an attribute, wouldn't I have to repeatedly update the counter in the for loop? — user2738698
– user2738698, Commented Jun 22, 2015 at 17:56
Python is pass by value. You get a copy of the thing, not a refrence. If you pass an object (a hash or list is an object), You can check if the elements in the list has changed. ex: pass aHash and look for aHash["watchVar"]. If aHash["watchVar"] , you can tell. — Robert Jacobs
– Robert Jacobs, Commented Jun 22, 2015 at 18:03

ron rothman · Accepted Answer · 2015-06-22 18:16:16Z

A raw int will not work, but as k4vin points, out, any other type of object that can be referenced, will.

We can demonstrate this with a list that contains the count, as k4vin did:

class Watcher(object):
    def __init__(self, to_watch):
        self.to_watch = to_watch

    def print_current_value(self):
        print self.to_watch

i = 0
watcher = Watcher(i)
watcher.print_current_value()
# prints 0
i += 3
watcher.print_current_value()
# still prints 0

l = [0]
watcher = Watcher(l)
watcher.print_current_value()
# prints [0]
l[0] += 3
watcher.print_current_value()
# prints [3]

But it's a little clunky to keep your count in a list, so one option is to roll your own simple counter, which you can then reference (as with the list):

class Counter(object):
    def __init__(self):
        self.count = 0

    def __add__(self, incr):
        self.count += incr

    def __str__(self):
        return str(self.count)

c = Counter()
watcher = Watcher(c)
watcher.print_current_value()
# prints 0
c += 3
watcher.print_current_value()
# hooray! prints 3

Kavin Eswaramoorthy · Accepted Answer · 2015-06-22 18:03:42Z

1

You can do that by wrapping the variable into a list as shown below since all the reference points to the same instance of the list

import time
class Count_Watcher:

    def __init__( self, to_watch ):

        self.to_watch = to_watch
        self.sleep_watch()

    def sleep_watch( self ):

        time.sleep( 5 )
        print( self.to_watch[0] )

line_counter = [0]
separate_thread_printer = Count_Watcher(line_counter)

for line in some_file:
    line_counter[0] += 1

answered Jun 22, 2015 at 18:03

Kavin Eswaramoorthy

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