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copied!<p>Using <code>traceit</code> to step through the program line-by-line:</p> <pre><code>import sys import linecache class SetTrace(object): ''' with SetTrace(monitor): ... ''' def __init__(self, func): self.func = func def __enter__(self): sys.settrace(self.func) return self def passit(self, frame, event, arg): return self.passit def __exit__(self, ext_type, exc_value, traceback): sys.settrace(self.passit) def traceit(frame, event, arg): ''' http://www.dalkescientific.com/writings/diary/archive/2005/04/20/tracing_python_code.html ''' if event == "line": lineno = frame.f_lineno filename = frame.f_globals["__file__"] if (filename.endswith(".pyc") or filename.endswith(".pyo")): filename = filename[:-1] name = frame.f_globals["__name__"] line = linecache.getline(filename, lineno) print("%s # %s:%s" % (line.rstrip(), name, lineno, )) return traceit def b(): for i in range(5): yield i x = (yield) print(x) def a(): g = b() next(g) for i in range(4): g.send(5) print(next(g)) with SetTrace(traceit): a() </code></pre> <p>we obtain</p> <pre><code>g = b() # __main__:44 next(g) # __main__:45 # runs b until you get to a yield for i in range(5): # __main__:38 yield i # __main__:39 # stop before the yield; resume a ^ for i in range(4): # __main__:46 g.send(5) # __main__:47 # resume b; (yield i) expression evals to 5 then thrown away x = (yield) # __main__:40 # stop before yield; resume a ^ print(next(g)) # __main__:48 # next(g) called; resume b; print not called yet print(x) # __main__:41 # next(g) causes (yield) to evaluate to None None for i in range(5): # __main__:38 yield i # __main__:39 # yield 1; resume a; `print(next(g))` prints 1 1 for i in range(4): # __main__:46 g.send(5) # __main__:47 # resume b; (yield i) expression evals to 5 then thrown away </code></pre> <p>The comments on the right-hand side (above) explain why Python prints <code>None</code> then <code>1</code>. If you get that far, I think it is clear why you get <code>None</code>, <code>2</code>, etc. -- it's the same story all over again with different values for <code>i</code>.</p> <p>The other scenario, where <code>x = (yield)</code> and <code>yield i</code> are reverse can be analyzed similarly.</p>

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