Python "raise from" usage
PythonPython 3.xSyntaxException HandlingPython Problem Overview
What's the difference between raise
and raise from
in Python?
try:
raise ValueError
except Exception as e:
raise IndexError
which yields
Traceback (most recent call last):
File "tmp.py", line 2, in <module>
raise ValueError
ValueError
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "tmp.py", line 4, in <module>
raise IndexError
IndexError
and
try:
raise ValueError
except Exception as e:
raise IndexError from e
which yields
Traceback (most recent call last):
File "tmp.py", line 2, in <module>
raise ValueError
ValueError
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "tmp.py", line 4, in <module>
raise IndexError from e
IndexError
Python Solutions
Solution 1 - Python
The difference is that when you use from
, the __cause__
attribute is set and the message states that the exception was directly caused by. If you omit the from
then no __cause__
is set, but the __context__
attribute may be set as well, and the traceback then shows the context as during handling something else happened.
Setting the __context__
happens if you used raise
in an exception handler; if you used raise
anywhere else no __context__
is set either.
If a __cause__
is set, a __suppress_context__ = True
flag is also set on the exception; when __suppress_context__
is set to True
, the __context__
is ignored when printing a traceback.
When raising from a exception handler where you don't want to show the context (don't want a during handling another exception happened message), then use raise ... from None
to set __suppress_context__
to True
.
In other words, Python sets a context on exceptions so you can introspect where an exception was raised, letting you see if another exception was replaced by it. You can also add a cause to an exception, making the traceback explicit about the other exception (use different wording), and the context is ignored (but can still be introspected when debugging). Using raise ... from None
lets you suppress the context being printed.
See the raise
statement documenation:
> The from
clause is used for exception chaining: if given, the second expression must be another exception class or instance, which will then be attached to the raised exception as the __cause__
attribute (which is writable). If the raised exception is not handled, both exceptions will be printed:
>
> >>> try:
> ... print(1 / 0)
> ... except Exception as exc:
> ... raise RuntimeError("Something bad happened") from exc
> ...
> Traceback (most recent call last):
> File "finally
clause: the previous exception is then attached as the new exception’s __context__
attribute:
>
> >>> try:
> ... print(1 / 0)
> ... except:
> ... raise RuntimeError("Something bad happened")
> ...
> Traceback (most recent call last):
> File "
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
> File "
Also see the Built-in Exceptions documentation for details on the context and cause information attached to exceptions.
Solution 2 - Python
PEP 3134, Exception Chaining and Embedded Tracebacks introduced chaining of exceptions (implicitly chained with explicit raise EXCEPTION
or implicit raise, and explicitly chained with explicit raise EXCEPTION from CAUSE
). Here are the relevant paragraphs to understand their usage:
> Motivation
> ----------
>
> During the handling of one exception (exception A), it is possible that another exception (exception B) may occur. In today’s Python (version 2.4), if this happens, exception B is propagated outward and exception A is lost. In order to debug the problem, it is useful to know about both exceptions. The __context__
attribute retains this information automatically.
>
> Sometimes it can be useful for an exception handler to intentionally re-raise an exception, either to provide extra information or to translate an exception to another type. The __cause__
attribute provides an explicit way to record the direct cause of an exception.
>
> […]
>
> Implicit Exception Chaining
> ---------------------------
>
> Here is an example to illustrate the __context__
attribute:
>
> python > def compute(a, b): > try: > a/b > except Exception, exc: > log(exc) > > def log(exc): > file = open('logfile.txt') # oops, forgot the 'w' > print >>file, exc > file.close() >
>
> Calling compute(0, 0)
causes a ZeroDivisionError
. The compute()
function catches this exception and calls log(exc)
, but the log()
function also raises an exception when it tries to write to a file that wasn’t opened for writing.
>
> In today’s Python, the caller of compute()
gets thrown an IOError
. The ZeroDivisionError
is lost. With the proposed change, the instance of IOError
has an additional __context__
attribute that retains the ZeroDivisionError
.
>
> […]
>
> Explicit Exception Chaining
> ---------------------------
>
> The __cause__
attribute on exception objects is always initialized to None
. It is set by a new form of the raise
statement:
>
> python > raise EXCEPTION from CAUSE >
>
> which is equivalent to:
>
> python > exc = EXCEPTION > exc.__cause__ = CAUSE > raise exc >
>
> In the following example, a database provides implementations for a few different kinds of storage, with file storage as one kind. The database designer wants errors to propagate as DatabaseError
objects so that the client doesn’t have to be aware of the storage-specific details, but doesn’t want to lose the underlying error information.
>
> python > class DatabaseError(Exception): > pass > > class FileDatabase(Database): > def __init__(self, filename): > try: > self.file = open(filename) > except IOError, exc: > raise DatabaseError('failed to open') from exc >
>
> If the call to open()
raises an exception, the problem will be reported as a DatabaseError
, with a __cause__
attribute that reveals the IOError
as the original cause.
>
> Enhanced Reporting
> ------------------
>
> The default exception handler will be modified to report chained exceptions. The chain of exceptions is traversed by following the __cause__
and __context__
attributes, with __cause__
taking priority. In keeping with the chronological order of tracebacks, the most recently raised exception is displayed last; that is, the display begins with the description of the innermost exception and backs up the chain to the outermost exception. The tracebacks are formatted as usual, with one of the lines:
>
> > The above exception was the direct cause of the following exception:
>
> or
>
> > During handling of the above exception, another exception occurred:
>
> between tracebacks, depending whether they are linked by __cause__
or __context__
respectively. Here is a sketch of the procedure:
>
> python > def print_chain(exc): > if exc.__cause__: > print_chain(exc.__cause__) > print '\nThe above exception was the direct cause...' > elif exc.__context__: > print_chain(exc.__context__) > print '\nDuring handling of the above exception, ...' > print_exc(exc) >
>
> […]
PEP 415, Implement Context Suppression with Exception Attributes then introduced suppression of exception contexts (with explicit raise EXCEPTION from None
). Here is the relevant paragraph to understand its usage:
> Proposal
> --------
>
> A new attribute on BaseException
, __suppress_context__
, will be introduced. Whenever __cause__
is set, __suppress_context__
will be set to True
. In particular, raise exc from cause
syntax will set exc.__suppress_context__
to True
. Exception printing code will check for that attribute to determine whether context and cause will be printed. __cause__
will return to its original purpose and values.
>
> There is precedence for __suppress_context__
with the print_line_and_file
exception attribute.
>
> To summarize, raise exc from cause
will be equivalent to:
>
> python > exc.__cause__ = cause > raise exc >
>
> where exc.__cause__ = cause
implicitly sets exc.__suppress_context__
.
So in PEP 415, the sketch of the procedure given in PEP 3134 becomes the following:
def print_chain(exc):
if exc.__cause__:
print_chain(exc.__cause__)
print '\nThe above exception was the direct cause...'
elif exc.__context__ and not exc.__suppress_context__:
print_chain(exc.__context__)
print '\nDuring handling of the above exception, ...'
print_exc(exc)