Flatten list from recursive function in Python

Question

I'm trying to find a way to flatten my list so it outputs:

['one', 'two', 'three', 'four', 'five', 'six', 'seven', 'eight', 'nine']

So far my recursive function returns a new nested list of values with matched keys in a dictionary (using an old parameter list and a dictionary) is as follows:

l = ['won', 'too', 'three', 'fore', 'five', 'six', 'seven', 'ate', 'nein']
d = dict(won='one', too='two', fore='four', ate='eight', nein='nine')

def subst(l,d):
     if len(l) == 1:
         if l[0] in d:
             return d[l[0]]
         else:
             return l[0]
     else:
         return [d[l[0]] if l[0] in d else l[0]] + [subst(l[1:],d)]

So far I've been getting:

['one', ['two', ['three', ['four', ['five', ['six', ['seven', ['eight', 'nine']]]]]]]]

Is there any way I can flatten the list while keeping the recursive integrity of the function?

VIKASH JAISWAL · Accepted Answer · 2014-02-12 09:46:21Z

1

map(lambda x: d[x] if x in d else x,l)

answered Feb 12, 2014 at 9:46

VIKASH JAISWAL

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1 Comment

Martin Tournoij Over a year ago

Perhaps you could add some description to your answer? Just posting a piece of code without explanation is rarely helpful.

Community · Accepted Answer · 2017-05-23 10:33:22Z

1

You could omit the [] around the recursive subst() call.

Alternatively, you could do

def subst(l, d):
    return [d.get(i, i) for i in l]

which just iterates over the list and replaces each item with the respective entry in d, if any, creating a new list with the results.

If you'd rather keep the list, you could do

def subst(l, d):
    for n, i in enumerate(l):
        l[n] = d.get(i, i)

In a meanwhile deleted answer, someone asked about the race condition in [d[x] for x in l if x in d].

The race condition - in the case of a threaded program - consists of the separation between if x in d and the d[x] access. If another thread deletes exactly that entry in the dict referenced by d, the test succeeds, but the access nevertheless fails.

edited May 23, 2017 at 10:33

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answered Feb 12, 2014 at 9:21

glglgl

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3 Comments

user2559679 Over a year ago

If I omit the [] around the recursive subst() call, I would get a TypeError: can only concatenate list (not "str") to list.

Karoly Horvath Over a year ago

@user2559679: because your code doesn't always return a list... fix it

user2559679 Over a year ago

@Karoly Horvath Eureka! Thank you! The break/base case in the function returns a string. Just fixed it with some [].

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