How to take an initial segment of the data structure graphs(n) ?

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Such object is called an iterator in the Python world, you will find tons of nice tutorials with examples on the web. For example, if you want a list containing the first 10 elements generated by it, just do:

sage: G = graphs(7)
sage: [G.next() for _ in range(10)]

If you want to iterate over its elements until you find an interesting one, you can do something like:

sage: for g in graphs(7):
....:     if very_interesting_property(g):
....:         do_something_interesting(g)

You can also produce an iterator selectinf the interesting ones:

sage: I = (g for g in graphs(7) if very_interesting_property(g))

and then do:

sage: for g in I:
....:      do_something_interesting(g)