The Hidden Power of the Python Runtime

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The Hidden Power of the Python Runtime Elizaveta Shashkova PyCon US 2020 Online Edition

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@lisa_shashkova About Me w 4PGUXBSF%FWFMPQFSBU+FU#SBJOT 1Z$IBSN*%& w 4U1FUFSTCVSH 3VTTJB w !MJTB@TIBTILPWB !2

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@lisa_shashkova !3 :PVBMSFBEZVTFJUFWFSZEBZ

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@lisa_shashkova Test Frameworks w VOJUUFTU w QZUFTU !4

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@lisa_shashkova AssertionError w VOJUUFTU w !5 Traceback (most recent call last): File “/file.py”, line 8, in test_value assert a == 2, "Wrong value!" AssertionError: Wrong value!

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@lisa_shashkova AssertionError w VOJUUFTU w QZUFTU !6 Traceback (most recent call last): File “/file.py”, line 8, in test_value assert a == 2, "Wrong value!" AssertionError: Wrong value! E AssertionError: Wrong value! E assert 1 == 2

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@lisa_shashkova Contents w 1ZUIPO3VOUJNF w (FUUJOH3VOUJNF*OGPSNBUJPO w %FWFMPQNFOU5PPMT !7

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@lisa_shashkova Contents w 1ZUIPO3VOUJNF w (FUUJOH3VOUJNF*OGPSNBUJPO w %FWFMPQNFOU5PPMT !8

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@lisa_shashkova !9 l&WFSZUIJOHJTBOPCKFDUJO1ZUIPOz

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@lisa_shashkova Python Objects w $PSFUZQFT w OVNCFST TUSJOHT DPMMFDUJPOT w 1SPHSBNVOJUPCKFDUT w GVODUJPOT DMBTTFT NPEVMFT !10

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@lisa_shashkova Created Explicitly a = 27 def get_answer(): return 42 class Person: def __init__(self, age): self.age = age !11

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@lisa_shashkova Python Objects w $SFBUFEFYQMJDJUMZ w WBSJBCMFT GVODUJPOT DMBTTFT NPEVMFT !12

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@lisa_shashkova Python Objects w $SFBUFEFYQMJDJUMZ w WBSJBCMFT GVODUJPOT DMBTTFT NPEVMFT w $SFBUFEJNQMJDJUMZ w stack frame DPEFPCKFDU !13

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@lisa_shashkova Stack Frame w 3FQSFTFOUTBQSPHSBNTDPQF w *OGPSNBUJPOBCPVUFYFDVUJPOTUBUF w $PSSFTQPOEJOHDPEFPCKFDU w -PDBMBOEHMPCBMWBSJBCMFT w 0UIFSEBUB !14

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@lisa_shashkova Stack Frame !15 def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1)) module bar() 1 2 3 4 5 6 7 8 9 10 11

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@lisa_shashkova Stack Frame !16 module bar() foo() def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1)) 1 2 3 4 5 6 7 8 9 10 11 def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1))

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@lisa_shashkova Stack Frame !17 module bar() foo() def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1)) 1 2 3 4 5 6 7 8 9 10 11 def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1))

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@lisa_shashkova Stack Frame !18 module bar() foo() def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1)) 1 2 3 4 5 6 7 8 9 10 11 def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1))

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@lisa_shashkova Stack Frame !19 module bar() def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1)) 1 2 3 4 5 6 7 8 9 10 11 def foo(n): n = n - 1 return n def bar(k): res = foo(k) return res * 2 print(bar(1))

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@lisa_shashkova Python Power w "WBJMBCMFPVUPGUIFCPY !20 module bar()

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@lisa_shashkova Python Power w "WBJMBCMFPVUPGUIFCPY w 4UBDLGSBNFPCKFDUTBDDFTT !21 module bar()

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@lisa_shashkova Contents w 1ZUIPO3VOUJNF w (FUUJOH3VOUJNF*OGPSNBUJPO w %FWFMPQNFOU5PPMT !22

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@lisa_shashkova Python Stack Frame •sys._getframe([depth]) •depth OVNCFSPGDBMMTCFMPXUIFUPQ •DVSSFOUGSBNF !23

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@lisa_shashkova Frame Object !24

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@lisa_shashkova Frame Object: Variables !25 w -PDBMWBSJBCMFT • frame.f_locals

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@lisa_shashkova Frame Object: Variables !26 w -PDBMWBSJBCMFT • frame.f_locals w (MPCBMWBSJBCMFT • frame.f_globals

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@lisa_shashkova Frame Object: Variables !27 w -PDBMWBSJBCMFT • frame.f_locals w (MPCBMWBSJBCMFT • frame.f_globals • locals() & globals()

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@lisa_shashkova Code Object !28 •frame.f_code -GSBNF`TBUUSJCVUF

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@lisa_shashkova Code Object !29 w 3FQSFTFOUTBDIVOLPGFYFDVUBCMFDPEF

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@lisa_shashkova Code Object !30 w 3FQSFTFOUTBDIVOLPGFYFDVUBCMFDPEF >>> c = compile('a + b', 'a.py', 'eval')

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0x104f8fc90, file "a.py", line 1>

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@lisa_shashkova Code Object !31 w 3FQSFTFOUTBDIVOLPGFYFDVUBCMFDPEF >>> c = compile('a + b', 'a.py', 'eval')

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0x104f8fc90, file "a.py", line 1>
>>> eval(c, {'a': 1, 'b': 2})
3

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@lisa_shashkova Code Object !32 •code.co_filename pMFOBNFXIFSFJUXBTDSFBUFE •code.co_nameOBNFPGGVODUJPOPSNPEVMF •code.co_varnames OBNFTPGWBSJBCMFT

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@lisa_shashkova Code Object !33 •code.co_filename pMFOBNFXIFSFJUXBTDSFBUFE •code.co_nameOBNFPGGVODUJPOPSNPEVMF •code.co_varnames OBNFTPGWBSJBCMFT •code.co_code DPNQJMFECZUFDPEF EJTBTTFNCMFXJUIdis.dis()

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@lisa_shashkova Frame Object !34 •frame.f_lineno -DVSSFOUMJOFOVNCFS •frame.f_trace -USBDJOHGVODUJPO •frame.f_back - QSFWJPVTGSBNF

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@lisa_shashkova Previous Frame !35 module bar() foo()

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@lisa_shashkova Previous Frame !36 Traceback (most recent call last): File "file.py", line 12, in print(bar(1)) File "file.py", line 8, in bar res = foo(k) File "file.py", line 2, in foo raise ValueError("Wrong value!") ValueError: Wrong value! module bar() foo()

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@lisa_shashkova Frame Object !37 w inspectNPEVMF

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@lisa_shashkova Frame Object !38 w inspectNPEVMF w )BOEMFGSBNFWBSJBCMFDBSFGVMMZ def handle_stackframe_without_leak(): frame = inspect.currentframe() try: # do something with the frame finally: del frame

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@lisa_shashkova Contents w 1ZUIPO3VOUJNF w (FUUJOH3VOUJNF*OGPSNBUJPO w %FWFMPQNFOU5PPMT !39

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@lisa_shashkova Contents w 1ZUIPO3VOUJNF w (FUUJOH3VOUJNF*OGPSNBUJPO w %FWFMPQNFOU5PPMT !40

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest !41

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@lisa_shashkova Exception Object w tb = e.__traceback__USBDFCBDLPCKFDU w tb.tb_frameGSBNFPCKFDU !42

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@lisa_shashkova AssertionError Variables !43 def vars_in_assert(e): tb = e.__traceback__ frame = tb.tb_frame code = frame.f_code line = tb.tb_lineno - code.co_firstlineno + 1 source = inspect.getsource(code)

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@lisa_shashkova AssertionError Variables !44 def get_vars_names(source, line): # get variables names with `ast` module def vars_in_assert(e): tb = e.__traceback__ frame = tb.tb_frame code = frame.f_code line = tb.tb_lineno - code.co_firstlineno + 1 source = inspect.getsource(code) for name in get_vars_names(source, line): pass

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@lisa_shashkova AssertionError Variables !45 def get_vars_names(source, line): # get variables names with `ast` module def vars_in_assert(e): tb = e.__traceback__ frame = tb.tb_frame code = frame.f_code line = tb.tb_lineno - code.co_firstlineno + 1 source = inspect.getsource(code) for name in get_vars_names(source, line): if name in frame.f_locals: var = frame.f_locals[name] print(f"{name} = {var}")

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@lisa_shashkova Usage !46 >>> try: assert a + b < 1 except AssertionError as e: vars_in_assert(e)

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@lisa_shashkova Usage !47 >>> try: assert a + b < 1 except AssertionError as e: vars_in_assert(e) File “/file.py", line 10, in foo assert a + b < 1 Variables Values: a = 1 b = 2

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest !48

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS !49

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@lisa_shashkova Debugger !50

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@lisa_shashkova Python Debugger w 5SBDJOHGVODUJPO w 'SBNFFWBMVBUJPOGVODUJPO !51

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@lisa_shashkova Tracing Function w tracefunc(frame, event, arg) w sys.settrace(tracefunc) - TFUUPDVSSFOUGSBNF w 4UPSFEJOBGSBNFframe.f_trace w %FCVHHFSBOBMZTFTFWFOUT !52

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@lisa_shashkova Frame Evaluation w frame_eval(frame, exc) w %FCVHHFSJOTFSUTCSFBLQPJOU`TDPEFJOUPDPEFPCKFDU !53

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@lisa_shashkova More About Debuggers w 1Z$PO64 w l%FCVHHJOHXJUI1ZUIPO#FUUFS 'BTUFS 4USPOHFSz !54

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@lisa_shashkova Access to Frame w tracefunc(frame, event, arg) w frame_eval(frame, exc) !55

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@lisa_shashkova Debugger: Location !56 w frame.f_code.co_filename BOE frame.f_lineno

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@lisa_shashkova Debugger: Variables !57 w frame.f_locals

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@lisa_shashkova Debugger: Frames !58 w frame.f_back

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS !59

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS w $PEFDPWFSBHF !60

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@lisa_shashkova Code Coverage w 4IPXTXIJDIMJOFTXFSFFYFDVUFE !61

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@lisa_shashkova coverage.py !62 w 5IFNPTUQPQVMBSDPEFDPWFSBHFMJCSBSZ

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@lisa_shashkova coverage.py !63 • tracefunc(frame, event, arg) • frame.f_code.co_filename BOE frame.f_lineno

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS w $PEFDPWFSBHF !64

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS w $PEFDPWFSBHF w 3VOUJNFUZQJOHUPPMT !65

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@lisa_shashkova Typing Tools w 1Z"OOPUBUFCZ%SPQCPY w .POLFZ5ZQFCZ*OTUBHSBN w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO  1Z$IBSN !66

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@lisa_shashkova Typing Tools w 1Z"OOPUBUFCZ%SPQCPY w .POLFZ5ZQFCZ*OTUBHSBN w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO  1Z$IBSN !67

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@lisa_shashkova Typing Tools w 1Z"OOPUBUFCZ%SPQCPY w .POLFZ5ZQFCZ*OTUBHSBN w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO  1Z$IBSN !68

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@lisa_shashkova Typing Tools w 1Z"OOPUBUFCZ%SPQCPY w .POLFZ5ZQFCZ*OTUBHSBN w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO  1Z$IBSN !69

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@lisa_shashkova Typing Tools w 1Z"OOPUBUFCZ%SPQCPY w .POLFZ5ZQFCZ*OTUBHSBN w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO  1Z$IBSN !70

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@lisa_shashkova Acces To Frame w 1Z"OOPUBUF .POLFZ5ZQF • sys.setprofile(profilefunc) • profilefunc(frame, event, arg) !71

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@lisa_shashkova Acces To Frame w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO1Z$IBSN w *OUFHSBUFEXJUI%FCVHHFS w "DDFTTUPBGSBNFPCKFDU !72

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@lisa_shashkova Collecting Types !73 def arg_names(co): nargs = co.co_argcount names = co.co_varnames return list(names[:nargs]) names = arg_names(frame.f_code)

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@lisa_shashkova Collecting Types !74 def arg_names(co): nargs = co.co_argcount names = co.co_varnames return list(names[:nargs]) names = arg_names(frame.f_code) locs = frame.f_locals objects = [locs[n] for n in names]

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@lisa_shashkova Typing Tools w 1Z"OOPUBUFCZ%SPQCPY w .POLFZ5ZQFCZ*OTUBHSBN w l$PMMFDU3VOUJNFJOGPSNBUJPOzJO  1Z$IBSN !75

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS w $PEFDPWFSBHF w 3VOUJNFUZQJOHUPPMT w !76

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@lisa_shashkova Concurrent Execution w 5ISFBET w "TZOD5BTLT !77

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@lisa_shashkova Python Threads !78 import threading def fun(): print("Hello!") t = threading.Thread(target=fun) t.start() t.join()

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@lisa_shashkova Synchronisation w -PDLGVOEBNFOUBMTZODISPOJTBUJPOPCKFDU !79 lock = threading.Lock() lock.acquire() # only one thread here lock.release() with lock: # equivalent

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@lisa_shashkova Deadlock w 8BJUJOHGPSSFTPVSDFTXIJDIDBO`UCFSFMFBTFE !80

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@lisa_shashkova Deadlock !81 def run1(): with lock1: with lock2: # do sth def run2(): with lock2: with lock1: # do sth else Thread(target=run1).start() Thread(target=run2).start()

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@lisa_shashkova Deadlock w 8BJUJOHGPSSFTPVSDFTXIJDIDBO`UCFSFMFBTFE w )BSEUPEFUFDUJOCJHQSPKFDUT !82

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@lisa_shashkova Thread States •sys._getframe() - GSBNFPCKFDUGPSDVSSFOUUISFBE •sys._current_frames() - UPQNPTUTUBDLGSBNFGPSFBDIUISFBE !83

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@lisa_shashkova Thread Handler w 1SJOUUSBDFCBDLTGPSUISFBETXJUIJOUFSWBM w )FMQUPpOEEFBEMPDLMPDBUJPO !84

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@lisa_shashkova Fault Handler •faulthandler.dump_traceback(file) •%VNQTUIFUSBDFCBDLTPGBMMUISFBETJOGPpMF •*NQMFNFOUFEOBUJWFMZ !85

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@lisa_shashkova Concurrent Execution w 5ISFBET w "TZOD5BTLT !86

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@lisa_shashkova Async Locks !87 alock = asyncio.Lock() alock.acquire() # only one task here alock.release() async with alock: # equivalent

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@lisa_shashkova Async Fault Handler •asyncio.all_tasks(loop) - BMMUIFSVOOJOHUBTLT •Task.get_stack() - MJTUPGTUBDLGSBNFTGPSUIJT5BTL !88

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@lisa_shashkova Async Fault Handler w *OBTFQBSBUFUISFBE !89 def dump_traceback_later(timeout, loop): while True: sleep(timeout) dump_traceback(loop, timeout)

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@lisa_shashkova Async Fault Handler w *OBTFQBSBUFUISFBE !90 def dump_traceback(loop): for task in asyncio.all_tasks(loop): task.print_stack() def dump_traceback_later(timeout, loop): while True: sleep(timeout) dump_traceback(loop, timeout)

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@lisa_shashkova Development Tools w "TTFSUJPO&SSPSJOpytest w %FCVHHFS w $PEFDPWFSBHF w 3VOUJNFUZQJOHUPPMT w "TZOD 'BVMU)BOEMFS !91

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@lisa_shashkova The Hidden Power of Runtime w 1ZUIPO3VOUJNFJTWFSZQPXFSGVM w &BTZBDDFTTUPTUBDLGSBNFBOEDPEFPCKFDUT w %FWFMPQNFOU5PPMT w QZUFTU %FCVHHFS $PEF$PWFSBHF   5ZQJOH*OGPSNBUJPO 'BVMU  )BOEMFS !92

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@lisa_shashkova Inspiration w 6TFFYJTUJOH3VOUJNF%FWFMPQNFOU5PPMT NPSFPGUFO w $SFBUFTPNFUIJOHOFX !93

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@lisa_shashkova Links w IUUQTHJUIVCDPN&MJ[BWFUB1Z3VOUJNF5SJDLT w IUUQTEPDTQZUIPOPSH w FMJ[BWFUBTIBTILPWB!KFUCSBJOTDPN w !MJTB@TIBTILPWB !94