perimosocordiae - 1 year ago 55

Python Question

I'm writing a ~~breadth~~ depth-first tree traversal function, and what I want to do is this:

`def traverse(node):`

yield node

for n in node.children:

yield_all traverse(n) # << if Python had a yield_all statement

The idea is to end up with a (flat) sequence of nodes in the tree.

Approach #1: (propagating yields)

`def traverse(node):`

yield node

for n in node.children:

for m in traverse(n):

yield m

Approach #2: (flattening sequences)

`def traverse(node):`

return itertools.chain([node],*(traverse(n) for n in node.children))

The first approach seems more clean, but I feel weird explicitly

`yield`

The second approach is terse and slightly dirty, but it matches what I would write in Haskell:

`traverse node = node : concatMap traverse (children node)`

So my question is: Which is better? Or am I missing a best 3rd option?

Answer

You are not the only one that pines for a `yield_all`

, see PEP380 (draft status for nearly 2 years, *sigh*).

This won't get me any Pythonic medal, but since you are clearly a functional-programming-guy, you could always write this (and feel at home):

```
import itertools
def concatMap(fun, it):
return itertools.chain.from_iterable(itertools.imap(fun, it))
def traverse(node):
return itertools.chain([node], concatMap(traverse, node.children))
```

Incidentally, I wrote a short Python implementation for PEP380 some time ago, using it it would look like this:

```
from pep380 import supergenerator, _from
@supergenerator
def traverse(node):
yield node
for n in node.children:
yield _from(traverse(n))
```

Of course, this is just a curiosity, if I expected someone else to read the code, I'd probably use your first approach (verbose as it is).

Source (Stackoverflow)