Usage

Pile d’appel

La pile d’appel (ou pile d’exécution ou call stack) mémorise les appels de fonctions. La premièrel ligne est le point d’entrée du programme. La suivante est la seconde fonction appelée. Si celle-ci en appelle une autre, une autre ligne est ajoutée et celle-ci demeure jusqu’à ce qu’elle est terminée son exécution. A chaque instant, la dernière ligne est la fonction en train de s’exécuter, les lignes précédentes définissent le chemin que l’ordinateur a suivi pour arriver jusque là.

<<<

import traceback
import sys


def foncA():
    print("foncA begin")
    foncB()
    print("foncA end")


def foncB():
    print("foncB begin")
    foncC()
    print("foncB end")


def foncC():
    print("foncC begin")
    try:
        raise Exception("erreur volontaire")
    except Exception:
        print("Erreur")
        print("\n".join(traceback.format_stack()))
    print("foncC end")


foncA()

>>>

    foncA begin
    foncB begin
    foncC begin
    Erreur
      File "somewhere/workspace/teachpyx/teachpyx_UT_39_std/_venv/lib/python3.9/site-packages/pyquickhelper/helpgen/process_sphinx_cmd.py", line 23, in <module>
        main()
    
      File "somewhere/workspace/teachpyx/teachpyx_UT_39_std/_venv/lib/python3.9/site-packages/pyquickhelper/helpgen/process_sphinx_cmd.py", line 19, in main
        run_sphinx_build(sys.argv)
    
      File "somewhere/workspace/teachpyx/teachpyx_UT_39_std/_venv/lib/python3.9/site-packages/pyquickhelper/helpgen/process_sphinx_cmd.py", line 15, in run_sphinx_build
        build_main(argv=argv[1:])
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/cmd/build.py", line 321, in main
        return build_main(argv)
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/cmd/build.py", line 285, in build_main
        app.build(args.force_all, args.filenames)
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/application.py", line 353, in build
        self.builder.build_update()
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 311, in build_update
        self.build(to_build,
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 328, in build
        updated_docnames = set(self.read())
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 435, in read
        self._read_serial(docnames)
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 456, in _read_serial
        self.read_doc(docname)
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/builders/__init__.py", line 512, in read_doc
        publisher.publish()
    
      File "/usr/local/lib/python3.9/site-packages/docutils/core.py", line 224, in publish
        self.document = self.reader.read(self.source, self.parser,
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/io.py", line 108, in read
        self.parse()
    
      File "/usr/local/lib/python3.9/site-packages/docutils/readers/__init__.py", line 76, in parse
        self.parser.parse(self.input, document)
    
      File "/usr/local/lib/python3.9/site-packages/sphinx/parsers.py", line 80, in parse
        self.statemachine.run(inputlines, document, inliner=self.inliner)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 169, in run
        results = StateMachineWS.run(self, input_lines, input_offset,
    
      File "/usr/local/lib/python3.9/site-packages/docutils/statemachine.py", line 233, in run
        context, next_state, result = self.check_line(
    
      File "/usr/local/lib/python3.9/site-packages/docutils/statemachine.py", line 445, in check_line
        return method(match, context, next_state)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 3024, in text
        self.section(title.lstrip(), source, style, lineno + 1, messages)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 325, in section
        self.new_subsection(title, lineno, messages)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 391, in new_subsection
        newabsoffset = self.nested_parse(
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 279, in nested_parse
        state_machine.run(block, input_offset, memo=self.memo,
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 195, in run
        results = StateMachineWS.run(self, input_lines, input_offset)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/statemachine.py", line 233, in run
        context, next_state, result = self.check_line(
    
      File "/usr/local/lib/python3.9/site-packages/docutils/statemachine.py", line 445, in check_line
        return method(match, context, next_state)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 2785, in underline
        self.section(title, source, style, lineno - 1, messages)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 325, in section
        self.new_subsection(title, lineno, messages)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 391, in new_subsection
        newabsoffset = self.nested_parse(
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 279, in nested_parse
        state_machine.run(block, input_offset, memo=self.memo,
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 195, in run
        results = StateMachineWS.run(self, input_lines, input_offset)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/statemachine.py", line 233, in run
        context, next_state, result = self.check_line(
    
      File "/usr/local/lib/python3.9/site-packages/docutils/statemachine.py", line 445, in check_line
        return method(match, context, next_state)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 2355, in explicit_markup
        nodelist, blank_finish = self.explicit_construct(match)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 2367, in explicit_construct
        return method(self, expmatch)
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 2104, in directive
        return self.run_directive(
    
      File "/usr/local/lib/python3.9/site-packages/docutils/parsers/rst/states.py", line 2154, in run_directive
        result = directive_instance.run()
    
      File "somewhere/workspace/teachpyx/teachpyx_UT_39_std/_venv/lib/python3.9/site-packages/pyquickhelper/sphinxext/sphinx_runpython_extension.py", line 568, in run
        out, err, context = run_python_script(script, comment=comment, setsysvar=p['setsysvar'],
    
      File "somewhere/workspace/teachpyx/teachpyx_UT_39_std/_venv/lib/python3.9/site-packages/pyquickhelper/sphinxext/sphinx_runpython_extension.py", line 229, in run_python_script
        exec(obj, globs, loc)
    
      File "", line 26, in <module>
    
      File "", line 25, in run_python_script_140112496841408
    
      File "", line 8, in foncA
    
      File "", line 13, in foncB
    
      File "", line 22, in foncC
    
    foncC end
    foncB end
    foncA end

Récupération de la pile d’appel

Le module traceback permet de récupérer la pile d’appels lorsqu’une exception survient.

<<<

def raise_exception():
    raise Exception("an error was raised")


try:
    insidefe()
except:
    exc_type, exc_value, exc_traceback = sys.exc_info()
    print("".join(traceback.format_tb(exc_traceback)))

>>>

      File "", line 7, in run_python_script_140112496871488

Il est possible de récupérer la liste des appels de fonctions avec la fonction extract_tb. Cette information est précieuse pour écrire un test qui vérifie qu’une erreur s’est bien produite à un endroit particulier, de détecter les cas particuliers comme les boucles infinies ou d’améliorer un message d’erreur en cas de besoin (lire How do I write Flask’s excellent debug log message to a file in production?).

Message d’erreur plus explicite

Lorsqu’une erreur se produit dans une librairie de Python, le message ne mentionne aucune information à propos du code qui l’a provoquée.

<<<

import math
ensemble = [1, 0, 2]
s = 0
for e in ensemble:
    s += math.log(e)

>>>

    
    [runpythonerror]
    
    Traceback (most recent call last):
        exec(obj, globs, loc)
      File "", line 8, in <module>
      File "", line 7, in run_python_script_140112496590784
    ValueError: math domain error

Typiquement dans ce cas précis, on ne sait pas quel est l’indice de l’élément qui a provoqué l’erreur. On utilise alors un mécanisme qui permet d’ajouter une erreur sans perdre les informations l’exception original

<<<

import math
ensemble = [1, 0, 2]
s = 0
for i, e in enumerate(ensemble):
    try:
        s += math.log(e)
    except Exception as exc:
        raise Exception("Issue with element {0}".format(i)) from exc

>>>

    
    [runpythonerror]
    
    Traceback (most recent call last):
      File "", line 8, in run_python_script_140112495973888
    ValueError: math domain error
    
    The above exception was the direct cause of the following exception:
    
    Traceback (most recent call last):
        exec(obj, globs, loc)
      File "", line 11, in <module>
      File "", line 10, in run_python_script_140112495973888
    Exception: Issue with element 1

La dernière partie de la dernière ligne est importante : from exc. langage garde ainsi la trace de la première exception.

Conventions

Erreur ou code d’erreur

A faire: terminer la section Erreur ou code d’erreur

parler aussi de coûts d’une exception, libération des ressources