2

If I write this: 

c = []

def cf(n):
    c = range (5)
    print c
    if any((i>3) for i in c) is True:
        print 'hello'

cf(1)

print c

Then I get:

[1, 2, 3, 4]
hello
[]

I'm really new to programming, so please explain it really simply, but how do I stop Python from forgetting what c is after the function has ended? I thought I could fix it by defining c before the function, but obviously that c is different to the one created just for the function loop.

In my example, I could obviously just write:

c = range (5)
def cf(n)

But the program I'm trying to write is more like this:

b = [blah]
c = []
def cf(n):
    c = [transformation of b]
    if (blah) is True:
       'loop' cf
    else:
cf(1)
g = [transformation of c that produces errors if c is empty or if c = b]

So I can't define c outside the function.

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7
  • 1
    put global c at the top of your function, and it will use the version defined at the top-level of your file, instead of creating a new variable inside the function. Commented Dec 4, 2014 at 23:36
  • 1
    global c use this in the cf Commented Dec 4, 2014 at 23:37
  • 3
    I will add the obligatory don't use global, there are almost always a better way to go. Commented Dec 4, 2014 at 23:50
  • @Patrik333 Note that although global c is a valid solution, it's a pretty poor one. Use return. Commented Dec 4, 2014 at 23:51
  • Note that you don't need the is True in your if statements. For tests that return a boolean anyway (like any() and all()), it is pure noise. For the cases where it does make a difference, if blah: is almost certainly what you mean over if blah is True:. Commented Dec 5, 2014 at 0:14

5 Answers 5

Reset to default
4

In python you can read global variables in functions, but you cant assigned to them by default. the reason is that whenever python finds c = it will create a local variable. Thus to assign to global one, you need explicitly specify that you are assigning to global variable.

So this will work, e.g.:

c = [1,2,3]

def cf(): 
    print(c) # it prints [1,2,3], it reads global c

However, this does not as you would expect:

c = [1,2,3]

def cf(): 
    c = 1 # c is local here.
    print(c) # it prints 1


cf()
print(c) # it prints [1,2,3], as its value not changed inside cf()

So to make c be same, you need:

c = [1,2,3]

def cf(): 
    global c
    c = 1  # c is global here. it overwrites [1,2,3]
    print(c) # prints 1


cf()
print(c) # prints 1. c value was changed inside cf()
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4 Comments

Awesome, this is exactly what I wanted! I'd click upvote but I need 15 rep... and I'd click 'answered' but apparently I have to wait 2 minutes. But yes, this is great, thanks.
@Patrik333 Thanks. Glad I could clarify the issue for you.
I actually have another related problem now - I guess I should make a new post?
@Patrik333 Yep, you can.
2

To summarise a few of these answers, you have 3 basic options:

  1. Declare the variable as global at the top of your function
  2. Return the local instance of the variable at the end of your function
  3. Pass the variable as an argument to your function
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Comments

2

You can also pass the array c into the function after declaring it. As the array is a function argument the c passed in will be modified as long as we don't use an = statement. This can be achieved like this:

def cf(n, c):
  c.extend(range(5))
  print c
  if any((i>3) for i in c) is True:
    print 'hello'

if __name__ == '__main__':
  c = []
  cf(1, c)
  print c

For an explanation of this see this

This is preferable to introducing global variables into your code (which is generally considered bad practice). ref

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Comments

1

Try this 

c = []

def cf(n):
  global c
  c = range (5)
  print c
  if any((i>3) for i in c) is True:
    print 'hello'

cf(1)

print c
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Comments

1

If you want your function to modify c then make it explicit, i.e. your function should return the new value of c. This way you avoid unwanted side effects:

def cf(n, b):
    """Given b loops n times ...

    Returns
    ------
    c: The modified value
    """
    c = [transformation of b]
    ...
    return c  # <<<<------- This

c = cf(1)
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