On Wed, 24 Jan 2007 21:48:38 +0100, Florian Lindner wrote:
Hello,
does python have static variables? I mean function-local variables that keep
their state between invocations of the function.
There are two ways of doing that (that I know of).
The simplest method is by having a mutable default argument. Here's an
example:
def foo(x, _history=[]):
print _history, x
_history.append(x)
>>foo(2)
[] 2
>>foo(3)
[2] 3
>>foo(5)
[2, 3] 5
Another method is to add an attribute to the function after you've created
it:
def foo(x):
print foo.last, x
foo.last = x
foo.last = None
>>foo(3)
None 3
>>foo(6)
3 6
>>foo(2)
6 2
But the most powerful method is using generators, which remember their
entire internal state between calls. Here's a simple example, one that
returns the integers 0, 1, 3, 6, 10, ...
def number_series():
increment = 1
n = 0
while True:
yield n # instead of return
n += increment
increment += 1
Notice that in this case there is no exit to the function: it loops
forever because the series goes on for ever. If you want to exit the
generator, just use a plain return statement (don't return anything), or
just exit the loop and fall off the end of the function.
This is how we might use it:
Create an iterator object from the generator function, and print the first
six values:
>>gen = number_series()
for i in range(6): print gen.next()
....
0
1
3
6
10
15
Sum the values from the current point up to 100:
>>s = 0
n = gen.next()
n
21
>>for x in gen:
.... if x >= 100:
.... break
.... n += x
....
>>n
420
Reset the iterator to the start:
>>gen = number_series()
For generators that terminate, you can get all the values in one go with
this:
everything = list(gen) # or everything = list(number_series())
but don't try this on my example, because it doesn't terminate!
--
Steven.
