See Timer Objects from threading.

How about

from threading import Timer

def timeout():
    print("Game over")

# duration is in seconds
t = Timer(20 * 60, timeout)
t.start()

# wait for time completion
t.join()

Should you want pass arguments to the timeout function, you can give them in the timer constructor:

def timeout(foo, bar=None):
    print('The arguments were: foo: {}, bar: {}'.format(foo, bar))

t = Timer(20 * 60, timeout, args=['something'], kwargs={'bar': 'else'})

Or you can use functools.partial to create a bound function, or you can pass in an instance-bound method.

I'd use a timedelta object.

from datetime import datetime, timedelta

...
period = timedelta(minutes=1)
next_time = datetime.now() + period
minutes = 0
while run == 'start':
    if next_time <= datetime.now():
        minutes += 1
        next_time += period

Your code's perfect except that you must do the following replacement:

minutes += 1 #instead of mins = minutes + 1

or

minutes = minutes + 1 #instead of mins = minutes + 1

but here's another solution to this problem:

def wait(time_in_seconds):
    time.sleep(time_in_seconds) #here it would be 1200 seconds (20 mins)

mins = minutes + 1

should be

minutes = minutes + 1

Also,

minutes = 0

needs to be outside of the while loop.

I want to create a kind of stopwatch that when minutes reach 20 minutes, brings up a dialog box.

All you need is to sleep the specified time. time.sleep() takes seconds to sleep, so 20 * 60 is 20 minutes.

import time
run = raw_input("Start? > ")
time.sleep(20 * 60)
your_code_to_bring_up_dialog_box()

# this is kind of timer, stop after the input minute run out.    
import time
min=int(input('>>')) 
while min>0:
    print min
    time.sleep(60) # every minute 
    min-=1  # take one minute 

import time 

...

def stopwatch(mins):
   # complete this whole code in some mins.
   time.sleep(60*mins)

...

import time
mintt=input("How many seconds you want to time?:")
timer=int(mintt)
while (timer != 0 ):
    timer=timer-1
    time.sleep(1)
    print(timer)

This work very good to time seconds.

You're probably looking for a Timer object:

Try having your while loop like this:

minutes = 0

while run == "start":
   current_sec = timer()
   #print current_sec
   if current_sec == 59:
      minutes = minutes + 1
      print ">>>>>>>>>>>>>>>>>>>>>", mins

import time
def timer(n):
    while n!=0:
        n=n-1
        time.sleep(n)#time.sleep(seconds) #here you can mention seconds according to your requirement.
        print "00 : ",n
timer(30) #here you can change n according to your requirement.

import time
def timer():
   now = time.localtime(time.time())
   return now[5]


run = raw_input("Start? > ")
while run == "start":
   minutes = 0
   current_sec = timer()
   #print current_sec
   if current_sec == 59:
      mins = minutes + 1
      print ">>>>>>>>>>>>>>>>>>>>>", mins

I was actually looking for a timer myself and your code seems to work, the probable reason for your minutes not being counted is that when you say that

minutes = 0

and then

mins = minutes + 1

it is the same as saying

mins = 0 + 1

I'm betting that every time you print mins it shows you "1" because of what i just explained, "0+1" will always result in "1".

What you have to do first is place your

minutes = 0

declaration outside of your while loop. After that you can delete the

mins = minutes + 1

line because you don't really need another variable in this case, just replace it with

minutes = minutes + 1

That way minutes will start off with a value of "0", receive the new value of "0+1", receive the new value of "1+1", receive the new value of "2+1", etc.

I realize that a lot of people answered it already but i thought it would help out more, learning wise, if you could see where you made a mistake and try to fix it.Hope it helped. Also, thanks for the timer.

from datetime import datetime
now=datetime.now()
Sec=-1
sec=now.strftime("%S")
SEC=0
while True:
    SEC=int(SEC)
    sec=int(sec)
    now=datetime.now()
    sec=now.strftime("%S")
    if SEC<sec:
        Sec+=1
    SEC=sec
print(Sec) #the timer is Sec