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python-3.xexceptionerror-handlingtopological-sort

CycleErrors in graphlib

I'm currently learning the graphlib of python 3.9. And I want to generate a CycleError but when I run the following code:

#!/usr/bin/python3

from graphlib import TopologicalSorter
ts = TopologicalSorter()
ts.add(2, 1)
ts.add(3, 2)
ts.add(4, 3)
ts.add(1, 4)

ts.static_order()

I don't get a CycleError. But if I change the code like this:

#!/usr/bin/python3

from graphlib import TopologicalSorter
ts = TopologicalSorter()
ts.add(2, 1)
ts.add(3, 2)
ts.add(4, 3)
ts.add(1, 4)

tuple(ts.static_order()) # why does this work with tuple or list ?

It works but why ?

Solution

  • TopologicalSorter.static_order() is generator; function body contains yield statement, then the return value of function call is iterator.

    Function body won't executed at all before very first attempt of extracting element from the iterator.