The default parameter of python functions

As I've said in a previous article, every thing in python is a object, maybe there are exceptions, but it's good enough to think in this way at the beginning.

So, function is a object.

Then, this deviates some thing we may take care of.

First, we should know, the definition of a function is in fact a executable statement.

e.g. def hello(names = []):

names.append('tom')

for i in names:

print 'hello'

the line def hello(): is executed. Creating an function object which can be used in the current region, but the body is not executed.

Then each time the function is called, the body is executed with this object.

So what to be careful of? The default parameter.

Because the object is only created once, the default parameter is generated in that, and if it's mutable, problems come.

As in the code above, you may see that more and more 'tom' gets into the list each time.

Then how to avoid it?

use None to initialize it or otherwise, you take care of it yourself.