Is it safe to read loop variable after loop (using Python 2)? My purpose is to check how many iterations in the loop are done.
Here is code to show the idea:
a=[1,2,3,4,5]
for i in range(len(a)):
if a[i] == 2:
break
print i # output is 1, is it safe to read i here?
Yes, it is fine to read it there. This is because when you create a for loop, internally, it has a mechanism to create the indexer for you (in your case being i) and then increases it one by one by assigning new value to it every time. Thus, you can use the i after the for loop. Thus after:
a=[1,2,3,4,5]
for i in range(len(a)):
if a[i] == 2:
break
i isn't really dropped. To drop i, you can use del keyword:
a=[1,2,3,4,5]
for i in range(len(a)):
if a[i] == 2:
break
del i #deleted here
print i # now this will give you error!
While to replace is, you just simply need to redefine it:
a=[1,2,3,4,5]
for i in range(len(a)):
if a[i] == 2:
break
i = [] #now this is a list, not an integer anymore
print i # now this will give you different result: []
Similarly, for example, if you create a list in an if block:
if i == 0: #suppose you really enter this block
a = [] #a is created here
a.append(b) #but a can be used here, assuming the previous if is really entered
This is just how Python works.
Some related posts:
def scope though, the variable will only last within the def
You could do it, and it would be fine to do so, since the value of i in this case only gets incremented before the execution of the internal loop. It would make this code a bit harder to read, but it would get the job done. The variable i is created in the same scope as the for loop. If the example code you gave is the entire file, then i would go out of scope when the file exists. If the code example you gave was inside a function, i would be created inside the function, then go out of scope when the function returns / exits.
An alternate way of doing this would be to use the for... in construction for loops in Python, together with the zip method, and keep a variable:
for element, i in zip(a, range(len(a)):
if element == 2:
index = i
Then at the end, you are setting a new variable called index and not relying on loop variables.
i exists after the loop, yes. It is created in the same scope as the loop is created in. In your example, it goes out of scope when the program finishes (assuming this is the whole program). If this program were inside a function, i would go out of scope when the function exited / returned.