How do I add the contents of an iterable to a set

Question

What is the  one       obvious way  to add all items of an iterable to an existing set

User · Answer

For the benefit of anyone who might believe e.g. that doing aset.add() in a loop would have performance competitive with doing aset.update(), here's an example of how you can test your beliefs quickly before going public:

>\python27\python -mtimeit -s"it=xrange(10000);a=set(xrange(100))" "a.update(it)"
1000 loops, best of 3: 294 usec per loop

>\python27\python -mtimeit -s"it=xrange(10000);a=set(xrange(100))" "for i in it:a.add(i)"
1000 loops, best of 3: 950 usec per loop

>\python27\python -mtimeit -s"it=xrange(10000);a=set(xrange(100))" "a |= set(it)"
1000 loops, best of 3: 458 usec per loop

>\python27\python -mtimeit -s"it=xrange(20000);a=set(xrange(100))" "a.update(it)"
1000 loops, best of 3: 598 usec per loop

>\python27\python -mtimeit -s"it=xrange(20000);a=set(xrange(100))" "for i in it:a.add(i)"
1000 loops, best of 3: 1.89 msec per loop

>\python27\python -mtimeit -s"it=xrange(20000);a=set(xrange(100))" "a |= set(it)"
1000 loops, best of 3: 891 usec per loop

Looks like the cost per item of the loop approach is over THREE times that of the update approach.

Using |= set() costs about 1.5x what update does but half of what adding each individual item in a loop does.

User · Answer

You can add elements of a list to a set like this    gt  gt  gt  foo   set range 0  4    gt  gt  gt  foo set  0  1  2  3    gt  gt  gt  foo update range 2  6    gt  gt  gt  foo set  0  1  2  3  4  5

User · Answer

Use list comprehension   Short circuiting the creation of iterable using a list for example      gt  gt  gt  x    1  2  3  4   gt  gt  gt    gt  gt  gt  k   x   iter      gt  gt  gt  k  lt listiterator object at 0x100517490 gt   gt  gt  gt  l    y for y in k   gt  gt  gt  l  1  2  3  4   gt  gt  gt    gt  gt  gt  z   Set  1 2    gt  gt  gt  z update l   gt  gt  gt  z set  1  2  3  4    gt  gt  gt      Edit  missed the set part of question

User · Answer

for item in items     extant set add item    For the record  I think the assertion that  There should be one-- and preferably only one --obvious way to do it   is bogus  It makes an assumption that many technical minded people make  that everyone thinks alike  What is obvious to one person is not so obvious to another    I would argue that my proposed solution is clearly readable  and does what you ask  I don t believe there are any performance hits involved with it--though I admit I might be missing something  But despite all of that  it might not be obvious and preferable to another developer

User · Answer

You can use the set   function to convert an iterable into a set  and then use standard set update operator      to add the unique values from your new set into the existing one    gt  gt  gt  a     1  2  3    gt  gt  gt  b     3  4  5    gt  gt  gt  a    set b   gt  gt  gt  a set  1  2  3  4  5

User · Answer

Just a quick update  timings using python 3       usr local bin python3 from timeit import Timer  a   set range 1  100000   b   list range 50000  150000    def one by one s  l       for i in l          s add i       def cast to list and back s  l       s   set list s    l   def update set s l       s update l    results are    one by one 10 184448844986036 cast to list and back 7 969255169969983 update set 2 212590195937082

[python] How do I add the contents of an iterable to a set?

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