I try to stop a .after function i tkinter in python 3.6, but have probems with namspace of variables. I get NameError: "name 'counter' is not defined" when I use this code:
I read this excellent post, from where I got the idea of using two buttons instead of one.
Use start and stop function with same button in Tkinter
def count(ac,rs):
if ac:
global counter
counter += 1
label.config(text=counter)
label.after(1000, count)
if rs:
counter = 0
counter += 1
count()
def start():
ac = True
rs=False
count(ac,rs)
def stop():
ac = False
label.configure(text='0')
rs = True
count(ac,rs)
Since I declare the variable counter in the count() function, I don't understand the NameError.
Related
I want to know if there are some cases where declaring global keyword is necessary in python.
Yes, there are some cases where global is neccessary.
Have a look at this code, which will work fine:
i = 42 # this is a global var
def f():
print(i)
But what if you would like to edit i (which is a global variable).
If you do this, you get an error:
i = 42 # this is a global var
def f():
i += 1 # this will not work
print(i)
We can only access i. If python compiles the function to bytecode it detects an assignment to a variable and it assumes it is a local variable. But this is not the case here (it is a global variable). Therefore if we also want to modify the global var i we must use the global keyword.
i = 42 # this is a global var
def f():
global i
i += 1 # this will change the global var without error
print(i)
When you have shared resources and want to make changes in global one.
a = 0
def add_five():
global a
a += 5
def remove_two():
global a
a -= 2
add_five() # a = 5
add_five() # a = 10
add_five() # a = 15
remove_two() # a = 13
I'm trying to design a counter which would be incremented each time an action is done. Something like this:
def action_on_accounts(self, accounts):
for account in accounts[9:]:
try:
self.browser.get(account)
time.sleep(5)
action_button = self.browser.find_element_by_xpath(u'//button[contains(#class, "Heart")]').click()
counter_var = self.count_actions(counter_var)
print(counter_var)
except selenium.common.exceptions.NoSuchElementException:
break
def count_actions(self, counter_var):
return counter_var + 1
def main(self):
counter_var = 0
(...)
This is throwing a UnboundLocalError: local variable 'counter_var' referenced before assignment
I have read that I have to declare counter_var as global inside function and did this:
def count_actions(self, counter_var):
global counter_var
return counter_var + 1
It was throwing SyntaxError: name 'counter_var' is parameter and global error.
So I tried this :
def count_actions(self):
global counter_var
return counter_var + 1
Calling it this way :
counter_var = self.count_actions()
print(counter_var)
And now I have NameError: name 'counter_var' is not defined....
Please assist
An alternative, simpler solution is to use python's built-in enumerate(); It exists so we don't have to make our own functions. You can then set the count equal to a global variable declared outside your functions
So the code would look like this:
counter_var = 0
def action_on_accounts(self, accounts):
for count, account in enumerate(accounts[9:]):
global counter_var
counter_var = count
print(counter_var)
def main(self):
global counter_var
counter_var = 0
(...)
You should consider defining counter_var as an attribute: self.counter_var. It'll be accessible throughout your class (assuming that's what's going on). You won't have to provide it explicitly as an argument in your functions/methods and no need to worry about global variables.
def action_on_accounts(self, accounts):
for account in accounts[9:]:
try:
self.browser.get(account)
time.sleep(5)
action_button = self.browser.find_element_by_xpath(u'//button[contains(#class, "Heart")]').click()
self.count_actions()
print(self.counter_var)
except selenium.common.exceptions.NoSuchElementException:
break
def count_actions(self):
self.counter_var += 1
def main(self):
self.counter_var = 0
(...)
You'll probably want to initialize self.counter_var with your class however.
Well if you really want to use a global variable (which I advice against) you can do it with the global keyword. You need that keyword to declare the variable. Example:
def action_on_accounts():
global counter_var #declare the variable
counter_var = count_actions(counter_var)
print(counter_var)
def count_actions(cv):
global counter_var #you do not have to declare the variable here since we are theoretically in the right scope
print(counter_var) #just to check that is global
return cv + 1 #no need to declare since it is a parameter
if __name__ == "__main__":
counter_var = 0
action_on_accounts() #1
action_on_accounts() #2
print(counter_var) #will return 2
I tested this in the IPython console (Python 3.6).
However I strongly recommend that you use attributes of classes to do the same effect (as in use self not global). Global variables may create bad code.
I know how to undo a drawing step in python turtle with turtle.undo(). But how can I make a Redo function ?
from tkinter import *
...#Just some other things
def undoStep():
turtle.undo()
def redoStep():
#What to put here
root.mainloop()
To make a redo function, you need to keep track of the each action, in a list actions for example. You will also need a variable i that tells you where you are in that list and each time you call undoStep, decrease i by one. Then redoStep has to perform the action actions[i]. Here is the code:
import turtle
actions = []
i = 0
def doStep(function, *args):
global i
actions.append((function, *args))
i += 1
function(*args)
def undoStep():
global i
if i > 0:
i -= 1
turtle.undo()
def redoStep():
global i
if i >= 0 and i < len(actions):
function, *args = actions[i]
function(*args)
i += 1
I'm rather new to Python and programming in general, so I apologise in advance if my terminology is incorrect.
hue_alert_delay = 0
def delays(name, delay):
global hue_alert_delay
if name == 'hue_alert_delay':
for i in range(0, delay):
hue_alert_delay += 1
time.sleep(1)
hue_alert_delay = 0
delays('hue_alert_delay', 60)
What I'm trying to achieve:
I would like the function to convert the 'name' parameter, which is a string input, into a pre-exiting variable, which will negate the need for multiple IF statements.
The above example includes only one IF statement, but for my project there will be a lot more and I would rather keep the function clean and simple.
This won't work, but it's what I'm trying to aim for:
hue_alert_delay = 0
def delays(name, delay):
global name
for i in range(0, delay):
name += 1
time.sleep(1)
hue_alert_delay = 0
delays('hue_alert_delay', 60)
Any assistance would be appreciated.
Use a dict:
values = {
'hue_alert_delay': 0
}
def delays(name, delay):
values[name] += 1
Whenever you feel like using "variable variables", what you most likely really want is a dict storing key-value associations. Yes, there are other ways to do literally what you want, but that soon leads to insane code.
Use a dictionary like so.
vars = {'hue_alert_delay':0}
def delays(name, delay):
for i in range(0, delay):
vars[name] += 1
time.sleep(1)
vars[name] = 0
You can also use globals()[name] but I won't recommend it.
Use a dictionary:
vars = {'hue_alert_delay':0}
def delays(name, delay):
for i in range(delay):
vars[name] += 1
time.sleep(1)
vars[name] = 0
delays('hue_alert_delay', 60)
I'm using ftplib to create a simple script to push out a file to multiple IP addresses, all set up as FTP servers. I wanted to display progress in the file upload process, but I'm having an issue. I use the callback argument of FTP.storbinary() and it works with something like this:
count = 0
def update(block):
count2 = str(count + 1)
print count2
However, if I try to do any arithmetic outside of a str() call, the program hangs. So the following doesn't work:
count = 0
def update(block):
count += 1
print count
Even wrapping count in a str() call doesn't work. It just hangs on the first call.
If you just try calling update yourself, instead of passing it to FTP.storbinary, you'll see the problem immediately:
>>> update('')
UnboundLocalError: local variable 'count' referenced before assignment
If you want to update a global variable, you have to mark it global explicitly:
def update(block):
global count
count += 1
print count
See the FAQ entry Why am I getting an UnboundLocalError when the variable has a value? and the following question What are the rules for local and global variables in Python?, and the docs on global, for more details.
A better way to solve this would be to write a class:
class FtpHandler(object):
def __init__(self):
self.count = 0
def update(self, block):
self.count += 1
print self.count
Then, to use it, you construct an instance of the class, and pass a bound method instead of a plain function to the FTP code. For example, instead of this:
ftp = ftplib.FTP(...)
# ...
ftp.storbinary(spam, eggs, callback=update)
… do this:
myhandler = FtpHandler()
ftp = ftplib.FTP(...)
# ...
ftp.storbinary(spam, eggs, callback=myhandler.update)
It doesn't just hang, it produces an Exception (specifically an UnboundLocalError). You're trying to modify a global variable inside of a function; to do this the variable must be declared global:
count = 0
def update(block):
global count
count += 1
print count
This is almost always a sign of bad design, in your case it would probably be better to use a class with an attribute:
class MyCallbackHandler(object):
def __init__(self):
self.count = 0
def update(self, block):
self.count += 1
#... etc.