Simple syntax for bringing a list element to the front in python?

Question

I have an array with a set of elements. I'd like to bring a given element to the front but otherwise leave the order unchanged. Do folks have suggestions as to the cleanest syntax for this?

This is the best I've been able to come up with, but it seems like bad form to have an N log N operation when an N operation could do.

 mylist = sorted(mylist, key=lambda x: x == targetvalue, reverse=True)

Your original code is very clear. Also, timsort (Python's internal sorting algorithm) works especially fast with long runs, such as yours ([False, False, False, False, ..., False, True, False, False ... False ]) — r3m0t
– r3m0t, Commented Nov 29, 2018 at 12:14
@jpp This question should not have been closed. It is a meaningfully special case of the linked "duplicate" question which is a lot more general. — Asclepius
– Asclepius, Commented Jan 6, 2023 at 17:33

Mike Hordecki · Accepted Answer · 2009-06-18 18:57:35Z

186

I would go with:

mylist.insert(0, mylist.pop(mylist.index(targetvalue)))

answered Jun 18, 2009 at 18:57

Mike Hordecki

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1 Comment

Anthony Petrillo Over a year ago

I combined Mike's answer and Alex's (below): "mylist.insert(0, mylist.pop(mylist.index(mylist[index_in_list])))" because I was working with tuples in the list and Alex's was giving me an error.

ZF007 · Accepted Answer · 2019-06-23 11:34:07Z

46

To bring (for example) the 6th element to the front, use:

mylist.insert(0, mylist.pop(5))

(python uses the standard 0 based indexing)

edited Jun 23, 2019 at 11:34

ZF007

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answered Jun 18, 2009 at 18:55

Alex Martelli

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2 Comments

Ulf Gjerdingen Over a year ago

and if we want to move an element called "hello!" and not knowing the index?

Elijas Dapšauskas Over a year ago

@UlfGjerdingen Use index = mylist.index(targetvalue)

proski · Accepted Answer · 2015-02-06 16:48:56Z

This requires just two list operations (no index): mylist.remove(targetvalue) mylist.insert(0, targetvalue)

Thomas Weigel · Accepted Answer · 2009-06-18 19:03:40Z

Note: the following code (and the sample code you offered) will put all matching elements at the front.

x = targetvalue for i in range(len(mylist)): if(mylist[i] == x): mylist = [mylist[i]] + mylist[:i] + mylist[i+1:]

For example, if mylist = [1, 2, 3, 4, 3] and x = 3, this will result in [3, 3, 1, 2, 4].

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