如何在Python中获取生成器执行的最后一行

Question

I am trying to get number of last line that has been executed by a generator in Python. I find a ways to get it in several cases:

• generator suspended by yield ,
• generator failed with an exception (not the StopIteration ),
• generator returned by explicit raising of the StopIteration .

But I am asking for an advice about how to get the line number in case of the StopIteration is raised automatically after the generator executed return statement or just reached last line of corresponding function.

A simplified code example I used is listed below.

from sys import exc_info

def gen1():
    print("1.1")
    yield
    print("1.2")
    raise StopIteration()

def gen2():
    print("2.1")
    yield
    print("2.2")
    return

def gen3():
    print("3.1")
    yield
    print("3.2")
    raise RuntimeError()

for gen in [ g for n, g in globals().items() if n.startswith("gen") ]:
    g = gen() # launch the generator
    while True:
        try:
            next(g)
        except StopIteration: # generator normally returned
            tb = exc_info()[2].tb_next
            if tb is None:
                print("How to get last line?")
            else:
                print("last line: %s" % tb.tb_frame.f_lineno)
            break
        except BaseException: # generator failure
            tb = exc_info()[2].tb_next
            print("last line: %s" % tb.tb_frame.f_lineno)
            break
        else:
            print("last line: %s" % g.gi_frame.f_lineno)

Execution of this code returns:

3.1
last line: 17
3.2
last line: 19
2.1
last line: 11
2.2
How to get last line?
1.1
last line: 5
1.2
last line: 7

Answer 1

The information you want is in the frame object.

But the frame object has already been garbage collected. The reference to it in g.gi_frame is set to None when the function returns.

The traceback won't help you, because the traceback comes from the machinery inside the interpreter after the function has returned, so it has nowhere to get it (and you've already seen that it doesn't even try).

---

But if you can change your code to grab a reference to the gi_frame object while the generator is running and hold onto it after it finishes, you'll keep that—and all of the other associated garbage—alive.

Which means f_lineno may be valid, as used in your last case that never happens. I don't know if there's any way to check when f_lineno is and isn't meaningful, but the traceback-generating code might know that if you dive into its source.

If not, f_lasti will definitely be the bytecode offset of the RETURN_VALUE op. Python of course doesn't guarantee anywhere that it doesn't do anything funky with the lasti before releasing the frame, but it doesn't in any version yet, and it's hard to see what reason there would be to do so in the future. So it should always be correct to use the information in f_code.co_lnotab to generate the line number for f_lasti . See the dis module for help on that. (If your Python is new enough, it exposes a findlinestarts that does it for you; if not, read the source of the module. Or write your own lnotab parser from the docs in the Objects directory of the interpreter source, if you want some real fun.)