I think it would be a good idea if Python tracebacks could be translated
into languages other than English - and it would set a good example.
For example, using French as my default local language, instead of
>>> 1/0
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ZeroDivisionError: integer division or modulo by zero
I might get something like
>>> 1/0
Suivi d'erreur (appel le plus récent en dernier) :
Fichier "<stdin>", à la ligne 1, dans <module>
ZeroDivisionError: division entière ou modulo par zéro
André
Hello,
What about adding in the json package the ability for an object to
provide a different object to serialize ?
This would be useful to translate a class into a structure that can be
passed to json.dumps
So, it __json__ is provided, its used for serialization instead of the
object itself:
>>> import json
>>> class MyComplexClass(object):
... def __json__(self):
... return 'json'
...
>>> o = MyComplexClass()
>>> json.dumps(o)
'"json"'
Cheers
Tarek
--
Tarek Ziadé | http://ziade.org
On Sat, Jul 31, 2010 at 10:12 PM, Antoine Pitrou <solipsis(a)pitrou.net> wrote:
> On Sat, 31 Jul 2010 13:50:16 +0200
> Tarek Ziadé <ziade.tarek(a)gmail.com> wrote:
>> On Sat, Jul 31, 2010 at 3:31 AM, Nick Coghlan <ncoghlan-Re5JQEeQqe8AvxtiuMwx3w(a)public.gmane.org> wrote:
>> > To be honest, there are actually some more features I would want to
>> > push for in ABCs (specifically, a public API to view an ABC's type
>> > registry, as well as a callback API to be notified of registration
>> > changes) before seriously proposing an official generic function
>> > implementation in the standard library.
>>
>> funny hazard, I was proposing to PEP 3319 authors about having the
>> _abc_registry attribute
>> somehow exposed.
>
> Rather than exposing the registry object itself (which is an
> implementation detail), how about exposing lookup operations on this
> registry?
There's a related problem here that ties into one of the complaints I
have with pkgutil.simplegeneric: because that decorator relies on MRO
traversal in order to obtain a reasonably efficient implementation, it
completely ignores any ABC registrations. That's fairly suboptimal,
since a comparable chain of "isinstance()" checks *will* respect ABC
registrations (it's just horrendously slow and doesn't scale, since
the worst-case number of checks increases linearly with the number of
branches in the if-elif chain).
So I think the idea of query methods in the abc module is a good way
to go. It allows the Python implementation freedom in choosing whether
to have separate type registries stored on the ABCs themselves, or
instead have global registries stored in the abc module. In
particular, it allows the interpreter to cache the transitive closure
of the ABC graph, such that an application can ask for the set of all
objects that implement a given ABC, as well as the set of all ABCs
that a given object implements.
Cheers,
Nick.
--
Nick Coghlan | ncoghlan(a)gmail.com | Brisbane, Australia
Hello,
I would like to propose the following PEP for feedback and review.
Permanent link to up-to-date version with proper HTML formatting:
http://www.python.org/dev/peps/pep-3151/
Thank you,
Antoine.
PEP: 3151
Title: Reworking the OS and IO exception hierarchy
Version: $Revision: 83042 $
Last-Modified: $Date: 2010-07-21 21:16:49 +0200 (mer. 21 juil. 2010) $
Author: Antoine Pitrou <solipsis(a)pitrou.net>
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 2010-07-21
Python-Version: 3.2 or 3.3
Post-History:
Resolution: TBD
Abstract
========
The standard exception hierarchy is an important part of the Python
language. It has two defining qualities: it is both generic and
selective. Generic in that the same exception type can be raised
- and handled - regardless of the context (for example, whether you are
trying to add something to an integer, to call a string method, or to write
an object on a socket, a TypeError will be raised for bad argument types).
Selective in that it allows the user to easily handle (silence, examine,
process, store or encapsulate...) specific kinds of error conditions
while letting other errors bubble up to higher calling contexts. For
example, you can choose to catch ZeroDivisionErrors without affecting
the default handling of other ArithmeticErrors (such as OverflowErrors).
This PEP proposes changes to a part of the exception hierarchy in
order to better embody the qualities mentioned above: the errors
related to operating system calls (OSError, IOError, select.error, and
all their subclasses).
Rationale
=========
Confusing set of OS-related exceptions
--------------------------------------
OS-related (or system call-related) exceptions are currently a diversity
of classes, arranged in the following subhierarchies::
+-- EnvironmentError
+-- IOError
+-- io.BlockingIOError
+-- io.UnsupportedOperation (also inherits from ValueError)
+-- socket.error
+-- OSError
+-- WindowsError
+-- mmap.error
+-- select.error
While some of these distinctions can be explained by implementation
considerations, they are often not very logical at a higher level. The
line separating OSError and IOError, for example, is often blurry. Consider
the following::
>>> os.remove("fff")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
OSError: [Errno 2] No such file or directory: 'fff'
>>> open("fff")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'fff'
The same error condition (a non-existing file) gets cast as two different
exceptions depending on which library function was called. The reason
for this is that the ``os`` module exclusively raises OSError (or its
subclass WindowsError) while the ``io`` module mostly raises IOError.
However, the user is interested in the nature of the error, not in which
part of the interpreter it comes from (since the latter is obvious from
reading the traceback message or application source code).
In fact, it is hard to think of any situation where OSError should be
caught but not IOError, or the reverse.
A further proof of the ambiguity of this segmentation is that the standard
library itself sometimes has problems deciding. For example, in the
``select`` module, similar failures will raise either ``select.error``,
``OSError`` or ``IOError`` depending on whether you are using select(),
a poll object, a kqueue object, or an epoll object. This makes user code
uselessly complicated since it has to be prepared to catch various
exception types, depending on which exact implementation of a single
primitive it chooses to use at runtime.
As for WindowsError, it seems to be a pointless distinction. First, it
only exists on Windows systems, which requires tedious compatibility code
in cross-platform applications (such code can be found in ``Lib/shutil.py``).
Second, it inherits from OSError and is raised for similar errors as OSError
is raised for on other systems. Third, the user wanting access to low-level
exception specifics has to examine the ``errno`` or ``winerror`` attribute
anyway.
Lack of fine-grained exceptions
-------------------------------
The current variety of OS-related exceptions doesn't allow the user to filter
easily for the desired kinds of failures. As an example, consider the task
of deleting a file if it exists. The Look Before You Leap (LBYL) idiom
suffers from an obvious race condition::
if os.path.exists(filename):
os.remove(filename)
If a file named as ``filename`` is created by another thread or process
between the calls to ``os.path.exists`` and ``os.remove``, it won't be
deleted. This can produce bugs in the application, or even security issues.
Therefore, the solution is to try to remove the file, and ignore the error
if the file doesn't exist (an idiom known as Easier to Ask Forgiveness
than to get Permission, or EAFP). Careful code will read like the following
(which works under both POSIX and Windows systems)::
try:
os.remove(filename)
except OSError as e:
if e.errno != errno.ENOENT:
raise
or even::
try:
os.remove(filename)
except EnvironmentError as e:
if e.errno != errno.ENOENT:
raise
This is a lot more to type, and also forces the user to remember the various
cryptic mnemonics from the ``errno`` module. It imposes an additional
cognitive burden and gets tiresome rather quickly. Consequently, many
programmers will instead write the following code, which silences exceptions
too broadly::
try:
os.remove(filename)
except OSError:
pass
``os.remove`` can raise an OSError not only when the file doesn't exist,
but in other possible situations (for example, the filename points to a
directory, or the current process doesn't have permission to remove
the file), which all indicate bugs in the application logic and therefore
shouldn't be silenced. What the programmer would like to write instead is
something such as::
try:
os.remove(filename)
except FileNotFound:
pass
Compatibility strategy
======================
Reworking the exception hierarchy will obviously change the exact semantics
of at least some existing code. While it is not possible to improve on the
current situation without changing exact semantics, it is possible to define
a narrower type of compatibility, which we will call **useful compatibility**,
and define as follows:
* *useful compatibility* doesn't make exception catching any narrower, but
it can be broader for *naïve* exception-catching code. Given the following
kind of snippet, all exceptions caught before this PEP will also be
caught after this PEP, but the reverse may be false::
try:
os.remove(filename)
except OSError:
pass
* *useful compatibility* doesn't alter the behaviour of *careful*
exception-catching code. Given the following kind of snippet, the same
errors should be silenced or reraised, regardless of whether this PEP
has been implemented or not::
try:
os.remove(filename)
except OSError as e:
if e.errno != errno.ENOENT:
raise
The rationale for this compromise is that careless (or "naïve") code
can't really be helped, but at least code which "works" won't suddenly
raise errors and crash. This is important since such code is likely to
be present in scripts used as cron tasks or automated system administration
programs.
Careful code should not be penalized.
Step 1: coalesce exception types
================================
The first step of the resolution is to coalesce existing exception types.
The extent of this step is not yet fully determined. A number of possible
changes are listed hereafter:
* alias both socket.error and select.error to IOError
* alias mmap.error to OSError
* alias IOError to OSError
* alias WindowsError to OSError
Each of these changes doesn't preserve exact compatibility, but it does
preserve *useful compatibility* (see "compatibility" section above).
Not only does this first step present the user a simpler landscape, but
it also allows for a better and more complete resolution of step 2
(see "Prerequisite" below).
Deprecation of names
--------------------
It is not yet decided whether the old names will be deprecated (then removed)
or all alternative names will continue living in the root namespace.
Deprecation of names from the root namespace presents some implementation
challenges, especially where performance is important.
Step 2: define additional subclasses
====================================
The second step of the resolution is to extend the hierarchy by defining
subclasses which will be raised, rather than their parent, for specific
errno values. Which errno values is subject to discussion, but a survey
of existing exception matching practices (see Appendix A) helps us
propose a reasonable subset of all values. Trying to map all errno
mnemonics, indeed, seems foolish, pointless, and would pollute the root
namespace.
Furthermore, in a couple of cases, different errno values could raise
the same exception subclass. For example, EAGAIN, EALREADY, EWOULDBLOCK
and EINPROGRESS are all used to signal that an operation on a non-blocking
socket would block (and therefore needs trying again later). They could
therefore all raise an identical subclass and let the user examine the
``errno`` attribute if (s)he so desires (see below "exception
attributes").
Prerequisite
------------
Step 1 is a loose prerequisite for this.
Prerequisite, because some errnos can currently be attached to different
exception classes: for example, EBADF can be attached to both OSError and
IOError, depending on the context. If we don't want to break *useful
compatibility*, we can't make an ``except OSError`` (or IOError) fail to
match an exception where it would succeed today.
Loose, because we could decide for a partial resolution of step 2
if existing exception classes are not coalesced: for example, EBADF could
raise a hypothetical BadFileDescriptor where an IOError was previously
raised, but continue to raise OSError otherwise.
The dependency on step 1 could be totally removed if the new subclasses
used multiple inheritance to match with all of the existing superclasses
(or, at least, OSError and IOError, which are arguable the most prevalent
ones). It would, however, make the hierarchy more complicated and
therefore harder to grasp for the user.
New exception classes
---------------------
The following tentative list of subclasses, along with a description and
the list of errnos mapped to them, is submitted to discussion:
* ``FileAlreadyExists``: trying to create a file or directory which already
exists (EEXIST)
* ``FileNotFound``: for all circumstances where a file and directory is
requested but doesn't exist (ENOENT)
* ``IsADirectory``: file-level operation (open(), os.remove()...) requested
on a directory (EISDIR)
* ``NotADirectory``: directory-level operation requested on something else
(ENOTDIR)
* ``PermissionDenied``: trying to run an operation without the adequate access
rights - for example filesystem permissions (EACCESS, optionally EPERM)
* ``BlockingIOError``: an operation would block on an object (e.g. socket) set
for non-blocking operation (EAGAIN, EALREADY, EWOULDBLOCK, EINPROGRESS);
this is the existing ``io.BlockingIOError`` with an extended role
* ``BadFileDescriptor``: operation on an invalid file descriptor (EBADF);
the default error message could point out that most causes are that
an existing file descriptor has been closed
* ``ConnectionAborted``: connection attempt aborted by peer (ECONNABORTED)
* ``ConnectionRefused``: connection reset by peer (ECONNREFUSED)
* ``ConnectionReset``: connection reset by peer (ECONNRESET)
* ``TimeoutError``: connection timed out (ECONNTIMEOUT); this could be re-cast
as a generic timeout exception, useful for other types of timeout (for
example in Lock.acquire())
This list assumes step 1 is accepted in full; the exception classes
described above would all derive from the now unified exception type
OSError. It will need reworking if a partial version of step 1 is accepted
instead (again, see appendix A for the current distribution of errnos
and exception types).
Exception attributes
--------------------
In order to preserve *useful compatibility*, these subclasses should still
set adequate values for the various exception attributes defined on the
superclass (for example ``errno``, ``filename``, and optionally
``winerror``).
Implementation
--------------
Since it is proposed that the subclasses are raised based purely on the
value of ``errno``, little or no changes should be required in extension
modules (either standard or third-party). As long as they use the
``PyErr_SetFromErrno()`` family of functions (or the
``PyErr_SetFromWindowsErr()`` family of functions under Windows), they
should automatically benefit from the new, finer-grained exception classes.
Library modules written in Python, though, will have to be adapted where
they currently use the following idiom (seen in ``Lib/tempfile.py``)::
raise IOError(_errno.EEXIST, "No usable temporary file name found")
Fortunately, such Python code is quite rare since raising OSError or IOError
with an errno value normally happens when interfacing with system calls,
which is usually done in C extensions.
If there is popular demand, the subroutine choosing an exception type based
on the errno value could be exposed for use in pure Python.
Possible objections
===================
Namespace pollution
-------------------
Making the exception hierarchy finer-grained makes the root (or builtins)
namespace larger. This is to be moderated, however, as:
* only a handful of additional classes are proposed;
* while standard exception types live in the root namespace, they are
visually distinguished by the fact that they use the CamelCase convention,
while almost all other builtins use lowercase naming (except True, False,
None, Ellipsis and NotImplemented)
An alternative would be to provide a separate module containing the
finer-grained exceptions, but that would defeat the purpose of
encouraging careful code over careless code, since the user would first
have to import the new module instead of using names already accessible.
Earlier discussion
==================
While this is the first time such as formal proposal is made, the idea
has received informal support in the past [1]_; both the introduction
of finer-grained exception classes and the coalescing of OSError and
IOError.
The removal of WindowsError alone has been discussed and rejected
as part of another PEP [2]_, but there seemed to be a consensus that the
distinction with OSError wasn't meaningful. This supports at least its
aliasing with OSError.
Moratorium
==========
The moratorium in effect on language builtins means this PEP has little
chance to be accepted for Python 3.2.
Possible alternative
====================
Pattern matching
----------------
Another possibility would be to introduce an advanced pattern matching
syntax when catching exceptions. For example::
try:
os.remove(filename)
except OSError as e if e.errno == errno.ENOENT:
pass
Several problems with this proposal:
* it introduces new syntax, which is perceived by the author to be a heavier
change compared to reworking the exception hierarchy
* it doesn't decrease typing effort significantly
* it doesn't relieve the programmer from the burden of having to remember
errno mnemonics
Exceptions ignored by this PEP
==============================
This PEP ignores ``EOFError``, which signals a truncated input stream in
various protocol and file format implementations (for example ``GzipFile``).
``EOFError`` is not OS- or IO-related, it is a logical error raised at
a higher level.
This PEP also ignores ``SSLError``, which is raised by the ``ssl`` module
in order to propagate errors signalled by the ``OpenSSL`` library. Ideally,
``SSLError`` would benefit from a similar but separate treatment since it
defines its own constants for error types (``ssl.SSL_ERROR_WANT_READ``,
etc.).
Appendix A: Survey of common errnos
===================================
This is a quick recension of the various errno mnemonics checked for in
the standard library and its tests, as part of ``except`` clauses.
Common errnos with OSError
--------------------------
* ``EBADF``: bad file descriptor (usually means the file descriptor was
closed)
* ``EEXIST``: file or directory exists
* ``EINTR``: interrupted function call
* ``EISDIR``: is a directory
* ``ENOTDIR``: not a directory
* ``ENOENT``: no such file or directory
* ``EOPNOTSUPP``: operation not supported on socket
(possible confusion with the existing io.UnsupportedOperation)
* ``EPERM``: operation not permitted (when using e.g. os.setuid())
Common errnos with IOError
--------------------------
* ``EACCES``: permission denied (for filesystem operations)
* ``EBADF``: bad file descriptor (with select.epoll); read operation on a
write-only GzipFile, or vice-versa
* ``EBUSY``: device or resource busy
* ``EISDIR``: is a directory (when trying to open())
* ``ENODEV``: no such device
* ``ENOENT``: no such file or directory (when trying to open())
* ``ETIMEDOUT``: connection timed out
Common errnos with socket.error
-------------------------------
All these errors may also be associated with a plain IOError, for example
when calling read() on a socket's file descriptor.
* ``EAGAIN``: resource temporarily unavailable (during a non-blocking socket
call except connect())
* ``EALREADY``: connection already in progress (during a non-blocking
connect())
* ``EINPROGRESS``: operation in progress (during a non-blocking connect())
* ``EINTR``: interrupted function call
* ``EISCONN``: the socket is connected
* ``ECONNABORTED``: connection aborted by peer (during an accept() call)
* ``ECONNREFUSED``: connection refused by peer
* ``ECONNRESET``: connection reset by peer
* ``ENOTCONN``: socket not connected
* ``ESHUTDOWN``: cannot send after transport endpoint shutdown
* ``EWOULDBLOCK``: same reasons as ``EAGAIN``
Common errnos with select.error
-------------------------------
* ``EINTR``: interrupted function call
Appendix B: Survey of raised OS and IO errors
=============================================
Interpreter core
----------------
Handling of PYTHONSTARTUP raises IOError (but the error gets discarded)::
$ PYTHONSTARTUP=foox ./python
Python 3.2a0 (py3k:82920M, Jul 16 2010, 22:53:23)
[GCC 4.4.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
Could not open PYTHONSTARTUP
IOError: [Errno 2] No such file or directory: 'foox'
``PyObject_Print()`` raises IOError when ferror() signals an error on the
`FILE *` parameter (which, in the source tree, is always either stdout or
stderr).
Unicode encoding and decoding using the ``mbcs`` encoding can raise
WindowsError for some error conditions.
Standard library
----------------
bz2
'''
Raises IOError throughout (OSError is unused)::
>>> bz2.BZ2File("foox", "rb")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory
>>> bz2.BZ2File("LICENSE", "rb").read()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: invalid data stream
>>> bz2.BZ2File("/tmp/zzz.bz2", "wb").read()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: file is not ready for reading
curses
''''''
Not examined.
dbm.gnu, dbm.ndbm
'''''''''''''''''
_dbm.error and _gdbm.error inherit from IOError::
>>> dbm.gnu.open("foox")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
_gdbm.error: [Errno 2] No such file or directory
fcntl
'''''
Raises IOError throughout (OSError is unused).
imp module
''''''''''
Raises IOError for bad file descriptors::
>>> imp.load_source("foo", "foo", 123)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 9] Bad file descriptor
io module
'''''''''
Raises IOError when trying to open a directory under Unix::
>>> open("Python/", "r")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 21] Is a directory: 'Python/'
Raises IOError or io.UnsupportedOperation (which inherits from the former)
for unsupported operations::
>>> open("LICENSE").write("bar")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: not writable
>>> io.StringIO().fileno()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
io.UnsupportedOperation: fileno
>>> open("LICENSE").seek(1, 1)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: can't do nonzero cur-relative seeks
Raises either IOError or TypeError when the inferior I/O layer misbehaves
(i.e. violates the API it is expected to implement).
Raises IOError when the underlying OS resource becomes invalid::
>>> f = open("LICENSE")
>>> os.close(f.fileno())
>>> f.read()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 9] Bad file descriptor
...or for implementation-specific optimizations::
>>> f = open("LICENSE")
>>> next(f)
'A. HISTORY OF THE SOFTWARE\n'
>>> f.tell()
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: telling position disabled by next() call
Raises BlockingIOError (inheriting from IOError) when a call on a non-blocking
object would block.
mmap
''''
Undex Unix, raises its own ``mmap.error`` (inheriting from EnvironmentError)
throughout::
>>> mmap.mmap(123, 10)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
mmap.error: [Errno 9] Bad file descriptor
>>> mmap.mmap(os.open("/tmp", os.O_RDONLY), 10)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
mmap.error: [Errno 13] Permission denied
Under Windows, however, it mostly raises WindowsError (the source code
also shows a few occurrences of ``mmap.error``)::
>>> fd = os.open("LICENSE", os.O_RDONLY)
>>> m = mmap.mmap(fd, 16384)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
WindowsError: [Error 5] Accès refusé
>>> sys.last_value.errno
13
>>> errno.errorcode[13]
'EACCES'
>>> m = mmap.mmap(-1, 4096)
>>> m.resize(16384)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
WindowsError: [Error 87] Paramètre incorrect
>>> sys.last_value.errno
22
>>> errno.errorcode[22]
'EINVAL'
multiprocessing
'''''''''''''''
Not examined.
os / posix
''''''''''
The ``os`` (or ``posix``) module raises OSError throughout, except under
Windows where WindosError can be raised instead.
ossaudiodev
'''''''''''
Raises IOError throughout (OSError is unused)::
>>> ossaudiodev.open("foo", "r")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory: 'foo'
readline
''''''''
Raises IOError in various file-handling functions::
>>> readline.read_history_file("foo")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory
>>> readline.read_init_file("foo")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 2] No such file or directory
>>> readline.write_history_file("/dev/nonexistent")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
IOError: [Errno 13] Permission denied
select
''''''
* select() and poll objects raise ``select.error``, which doesn't inherit from
anything (but poll.modify() raises IOError);
* epoll objects raise IOError;
* kqueue objects raise both OSError and IOError.
signal
''''''
``signal.ItimerError`` inherits from IOError.
socket
''''''
``socket.error`` inherits from IOError.
sys
'''
``sys.getwindowsversion()`` raises WindowsError with a bogus error number
if the ``GetVersionEx()`` call fails.
time
''''
Raises IOError for internal errors in time.time() and time.sleep().
zipimport
'''''''''
zipimporter.get_data() can raise IOError.
References
==========
.. [1] "IO module precisions and exception hierarchy"
http://mail.python.org/pipermail/python-dev/2009-September/092130.html
.. [2] Discussion of "Removing WindowsError" in PEP 348
http://www.python.org/dev/peps/pep-0348/#removing-windowserror
Copyright
=========
This document has been placed in the public domain.
..
Local Variables:
mode: indented-text
indent-tabs-mode: nil
sentence-end-double-space: t
fill-column: 70
coding: utf-8
End:
Hello all,
What if str.split could take an empty separator?
>>> 'banana'.split('')
['b', 'a', 'n', 'a', 'n', 'a']
I know this can be done with:
>>> list('banana')
['b', 'a', 'n', 'a', 'n', 'a']
I think that, semantically speaking, it would make sens to split where
there are no characters (in between them). Right now you can join from
an empty string:
''.join(['b', 'a', 'n', 'a', 'n', 'a'])
So why can't we split from an empty string?
This wouldn't introduce any backwards incompatible changes as
str.split currently can't have an empty separator:
>>> 'banana'.split('')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: empty separator
I would love to see my banana actually split. :)
Regards,
--
Alex
twitter.com/alexconrad
Can someone help me and tell why this does not work.
prys=0
tp=0
pr=0
f = open('C:\Documents and Settings\ZU1TN\Desktop\Nommers\K55.txt', 'r')
pr=f.readline()
prys =int(prys)
tp =int(tp)
pr =int(pr)
tp=pr-prys
f.close
tp=str(tp)
print tp
raw_input()
THX
Recently I've been wondering why __contains__ casts all of it's
returns to be boolean values. Specifically I'd like to propose that
__contains__'s return values be passed directly back as the result of
the `in` operation. As a result I'd further propose the introduction
of __not_contains__, which is the `not in` operator. The primary
usecase for this is something like an expression recorder. For
example in SQLAlchemy one can do:
User.id == 3, but not User.id in SQLList([1, 2, 3]), because it returns a
bool always. __not_contains__ is needed to be the analog of this, as
it cannot be merely be a negation of __contains__ when it's returning
a non-bool result.
There should be no backwards compatibility issues to making
__contains__ return non-bools, unless there is code like:
x = y in foo
assert type(x) is bool
However, for __not_contains__ I'd propose the default implementation be:
def __not_contains__(self, val):
x = val in self
if type(x) is not bool:
raise TypeError("%s returned a non-boolean value from
__contains__ and didn't provide an implementation of
__not_contains__")
return not x
This is not perfect (and it's at odds with the fact that __ne__
doesn't return not self == other), but it seems to allow both the
desired flexibility and backwards compatibility.
I'm not sure if this is something that'd be covered by the language
moratorium, but if not I can try putting together a patch for this.
Alex
--
"I disapprove of what you say, but I will defend to the death your
right to say it." -- Voltaire
"The people's good is the highest law." -- Cicero
"Code can always be simpler than you think, but never as simple as you
want" -- Me
Looking at PEP 380 (http://www.python.org/dev/peps/pep-0380/), the
need for yield forwarding syntax comes from the inability to delegate
yielding functionality in the usual way. For example, if you have a
recurring pattern including yields, like this (this is a toy example,
please don't take it for more than that):
if a:
yield a
if b:
yield b
you cannot do the extract function refactoring in the usual way:
def yield_if_true(x):
if x:
yield x
yield_if_true(a)
yield_if_true(b)
because yield_if_true would become a generator.
PEP 380 addresses this by making the workaround - "for x in
yield_if_true(a): yield x" - easier to write.
But suppose you could address the source instead? Suppose you could
write yield_if_true in such a way that it did not become a generator
despite yielding?
Syntactically this could be done with a yielding *function* in
addition to the yield statement/expression, either as a builtin or a
function in the sys module. Let's call it 'yield_' , for lack of a
better name. The function would yield the nearest generator on the
call stack.
Now the example would work with a slight modifiction:
def yield_if_true(x):
if x:
yield_(x)
yield_if_true(a)
yield_if_true(b)
The real benefits from a yield_ function come with recursive
functions. A recursive tree traversal that yields from the leaf nodes
currently suffers from a performance penalty: Every yield is repeated
as many times as the depth of the tree, turning a O(n) traversal
algorithm into an O(n lg(n)) algorithm. PEP 380 does not change that.
But a yield_ function could be O(1), regardless of the forwarding
depth.
To be fair, a clever implementation might be able to short-circuit a
'yield from' chain and achieve the same thing.
Two main drawbacks of yield_:
- Difficulty of implementation. Generators would need to keep an
entire stack branch alive, instead of merely a single frame, and if
that somehow affects the efficiency of simple generators, that would
be bad.
- 'the nearest generator on the call stack' is sort of a dynamic
scoping thing, which has its problems. For example, if you forget
to make the relevant function a generator (the "if 0: yield None"
trick might have been needed but was forgotten), then the yield
would trickle up to some random generator higher up, with confusing
results. And if you use yield_ in a callback, well, let's just say
that an interesting case too.
All the same, if a yield_ function is practical, I think it is a
better way to address the problems that motivate PEP 380.
I'm guessing you could implement 'yield from' as a pure-Python
function using yield_, making yield_ strictly more powerful, although
I couldn't say for sure as I haven't studied the enhanced generator
protocol.
regards, Anders
Hi,
I have downloaded a package from PyPI that uses distutils setup.py.
When I run it with -h it shows options for building and installing, but
does not appear to have an option for installation in my per user site
packages directory (see PEP 370).
I think it would be useful to add a "--local" option to setup.py that
would install into the per site package directory. This would allow
people to keep their Linux distros pristine while still being able to
install packages that their distros don't have. (Or is there such an
option that I've missed?)
(In my particular case it wasn't a problem; I just built it and moved
it.)
--
Mark Summerfield, Qtrac Ltd, www.qtrac.eu
C++, Python, Qt, PyQt - training and consultancy
"Programming in Python 3" - ISBN 0321680561
http://www.qtrac.eu/py3book.html
