* johngilbrough:
I cannot make sense of what's happening here ... I'm getting the
following error:
initializing last modified time
/home/john/Dropbox/Projects/python/scripts/src 29
referencing last modified time
/home/john/Dropbox/Projects/python/scripts/src 29
referencing last modified time
Traceback (most recent call last):
File "/home/john/Dropbox/Projects/python/scripts/src/file-watch.py",
line 42, in <module>
time.sleep(10000)
File "/home/john/Dropbox/Projects/python/scripts/src/file-watch.py",
line 18, in handler
if modifiedTime <> lastModifiedTime:
UnboundLocalError: local variable 'lastModifiedTime' referenced before
assignment
From this logic:
#!/usr/bin/python
def watchFile(filePath, callback):
###
# calls callback whenever file is changed
###
import fcntl
import os
import signal
print "initializing last modified time"
lastModifiedTime = os.path.getmtime(filePath)
def handler(signum, frame):
## this gets called twice whenever a file changes
print filePath + " " + str(signum)
modifiedTime = os.path.getmtime(filePath)
print "referencing last modified time"
if modifiedTime <> lastModifiedTime:
lastModifiedTime = modifiedTime
callback()
Since 'handler' has an assignment to 'lastModifiedTime' that name becomes the
name of a local variable. It's not the execution of the assignment that creates
the variable. It's the /presence/ of the assignment (this helps the compiler
generate code that allocates all local variables on entry to the function).
There are a couple of ways around.
(1)
At least in Py3 you can declare the variable as 'global', like this:
global lastModifiedTime
within the function.
(2)
Or, you can apply some indirection, which is nearly always a solution to any
computer science and programming problem, and declare your variable like so:
class Object: pass
g = Object()
g.lastModifiedTime = os.path.getmtime( filePath )
Then when you assign to 'g.lastModifiedTime' in 'handler' you're not creating a
variable, because you're assigning to an attribute of an object.
(3)
Best is however to recognize that you have some state (your variable) and some
operations on that state (your callback), and that that is what objects are all
about. I.e. wrap your logic in a class. Then 'lastModifiedTime' becomes an
instance attribute, and 'handler' becomes a method.
It doesn't matter that there will only ever be one object (instance) of that
class.
Classes were meant for just this sort of thing, state + operations.
Cheers & hth.,
- Alf
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