Bill Allen wrote:
> I am not understanding the following code (I did not write it). It
> demonstrates walking a tree-like data structure using recursion. It does
> run and produces reasonable output. I particularly do not understand the
> "traverse.level" statements. Can anyone give me an idea how this is
> working
> and the principles? I would like understand recursive calls in Python
> better as I have not used the technique previously.
> def traverse(data):
> print(' ' * traverse.level + data['text'])
> for kid in data['kids']:
> traverse.level += 1
> traverse(kid)
> traverse.level -= 1
>
> if __name__ == '__main__':
> traverse.level = 1
> traverse(data)
Functions are objects in python; you can tuck arbitrary attributes onto
them, and the above uses a function attribute instead of a global variable.
A more standard way to write the above would be
def traverse(data):
global level
print(' ' * level + data['text'])
for kid in data['kids']:
level += 1
traverse(kid)
level -= 1
if __name__ == '__main__':
level = 1
traverse(data)
What it does:
* call traverse with the outermost dictionary
* print the data["text"] value indented by the current global level.
* iterate over the data["kids"] list of dictionaries
* for each entry in that list
- increment indentation level
- invoke traverse which will print the data["text"] value.
(Remember that traverse was called with the current value of kid, so
in terms of the outer traverse() the inner traverse() is printing
kid["text"]) Process the kid's kids in the same way.
- decrement the indentation level
However using global variables is generally a bad practice.
It is easy to leave them in an inconsistent state, and if you are using
multiple threads (i. e. invoke traverse() a second time while the first call
hasn't finished) you'll end up with a big mess.
I would therefore write the traverse function as
def traverse(data, level):
print(' ' * level + data['text'])
for kid in data['kids']:
traverse(kid, level+1)
if __name__ == "__main__":
traverse(data, 1)
which I think may also be easier to understand.
Peter
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