Nesting Instinct

challenge flatten python tuple

I figured I'd start a series of Python challenges in my blog. Whenever I've posted questions such as these in the past, the discussion that follows is always very entertaining! A discussion arose today at work about the best way to flatten a tuple consisting of values and […]

I figured I'd start a series of Python challenges in my blog. Whenever I've posted questions such as these in the past, the discussion that follows is always very entertaining!

A discussion arose today at work about the best way to flatten a tuple consisting of values and other tuples (which may be nested arbitrarily). So the tuple (1, (1, 2, (1, 2, 3), 3)) would become (1, 1, 2, 1, 2, 3, 3). Now there was actually code implemented which was functional, efficient and easy to read – but where is the fun in that?

I figured I could re-write it in a more terse manner, and here is what I came up with:

def flatten(tpl):
    return eval(repr(tpl).replace('(', '').replace(')', ''))

t= (1, (1, 2, (1, 2, 3), 3))
print flatten(t)

This flatten function would actually work in the context of our app, but I would never use it in production code. My challenge today, is to tell me why this code should never be used!

pythonista —

You could have string values (for example) with parenthesis in them. These values would be changed.

Reply to pythonista

Steve Holden —

A further reason is that you have no idea what the repr of the object looks like, since you don't even examine its type …

One must hope nobody starts devising malicious objects with exploitive repr()s.

Cute idea, though.

Reply to Steve Holden

Euan Goddard —

I can't believe you had the audacity to post that! Very amusing though. Let me know when you can properly do it in 2 lines

Reply to Euan Goddard

WIll McGugan —

How about this, Euan…

 def flatten(tpl):
    return sum(map(flatten, tpl), ()) if type(tpl) is tuple else (tpl,)
    
t= (1, (1, 2, (1, 2, 3), 3))
print flatten(t)

Not quite as concise as the the other flatten, but less of a crime against Python!

Reply to WIll McGugan

jdm —

2euan: is this “properly”?

 def flatten(tpl):
 return reduce(lambda x,y: x+flatten(y),tpl,()) if type(tpl) is tuple else (tpl,)

Reply to jdm

Roberto —

A safe variant of your solution, assuming the values are integers:

def flatten(tpl):
return tuple( map(int, repr(tpl).replace('(', ‘').replace(’)', ‘').split(’, ')) )

Reply to Roberto

Roberto —

Posting again, sorry:

 def flatten(tpl):
        return tuple( map(int, repr(tpl).replace('(', '').replace(')', '').split(', ')) )

Reply to Roberto

jdm —

oops, didn't see Will's, which is cleaner.

Reply to jdm

Paddy3118 —

Since I've been glancing at the Y combinator

 >>> t= (1, (1, 2, (1, 2, 3), 3))
>>> _smash = lambda f: lambda t: sum(map(f, t), ()) if type(t) is tuple else (t,)
>>> Y = lambda f: (lambda x: x(x))(lambda y: f(lambda z: y(y)(z)))
>>> # From two non recursive functions:
>>> smash = Y(_smash)
>>> smash(t)
(1, 1, 2, 1, 2, 3, 3)
>>>

Remember: Don't do this at home folks :-)

- Paddy.

Reply to Paddy3118

apoirier —

One liner variation, working with any iterable:

flatten = lambda t: sum(map(flatten, t), ()) if hasattr(t, ‘__iter__’) else (t,)

Reply to apoirier

Eno —

 from itertools import chain
flatten = lambda x :chain(*(flatten(v) if type(v) is tuple else (v,) for v in x))
t = (1, (1, 2, (1, 2, 3), 3))
print list(flatten(t))

I recommend a visit to http://www.challenge-you.com if you enjoy those kind of challenges.

Reply to Eno

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