testing.py 文件源码

def __enter__(self):
        clean_warning_registry()  # be safe and not propagate state + chaos
        warnings.simplefilter('always')
        if self._entered:
            raise RuntimeError("Cannot enter %r twice" % self)
        self._entered = True
        self._filters = self._module.filters
        self._module.filters = self._filters[:]
        self._showwarning = self._module.showwarning
        if self._record:
            self.log = []

            def showwarning(*args, **kwargs):
                self.log.append(warnings.WarningMessage(*args, **kwargs))
            self._module.showwarning = showwarning
            return self.log
        else:
            return None