Mailman 3 March 2011 - Python-ideas

Serialization of custom classes.
by Jakob Bowyer March 30, 2011

March 30, 2011

""" Currently json dumping an object is sketchy because of the need to serialise. E.g. """ import json class Something(object): def __init__(self, arg1): self.arg1 = arg1 def __str__(self): return str(self.arg1) def __repr__(self): return str(self.arg1) def double(self): return self.arg1 * 2 test = Something(42) #json.dumps(test) """This ofc raises a TypeError as shown here ------------------------------------------------------------ … [View More]Traceback (most recent call last): File "<ipython console>", line 1, in <module> File "C:\Python26\Lib\site-packages\spyderlib\widgets\externalshell\startup.py", line 122, in runfile execfile(filename, glbs) File "C:\infobarb\test.py", line 22, in <module> json.dumps(test) File "C:\Python26\lib\json\__init__.py", line 230, in dumps return _default_encoder.encode(obj) File "C:\Python26\lib\json\encoder.py", line 367, in encode chunks = list(self.iterencode(o)) File "C:\Python26\lib\json\encoder.py", line 317, in _iterencode for chunk in self._iterencode_default(o, markers): File "C:\Python26\lib\json\encoder.py", line 323, in _iterencode_default newobj = self.default(o) File "C:\Python26\lib\json\encoder.py", line 344, in default raise TypeError(repr(o) + " is not JSON serializable") TypeError: 42 is not JSON serializable My suggested fix and an idea used by several others could be to add a __json__ method to objects, this method would be tried by json.dumps() before it tries to serialise the argument. It should return all of the instance variables in a json format and any other infomation considered to be correct for the dump. This could be extended to provide a __json__ and a __jsons__ format where the latter returns a string format of the __json__ serialzation an example of which (crudely constructed ofc) is below. But for now I consider __json__ to return a string serialized for json dumping. """ import json def customjsondumps(obj): try: return json.dumps(obj.__json__()) except AttributeError: raise AttributeError('Object has no __json__ method.') """There are ofc several ideas for the __json__ method e.g.""" def __json__(self): """This method returns a serialized string for json.dumps""" return self.__dict__ """Or returning some fancy constructed dict, list or other serialised form for dumping into json.""" class Something(object): def __init__(self, arg1): self.arg1 = arg1 def __str__(self): return str(self.arg1) def __repr__(self): return str(self.arg1) def double(self): return self.arg1 * 2 def __json__(self): ret = self.__dict__.copy() for key, item in ret.iteritems(): ret[key] = item.__repr__() return ret test = Something(41) [View Less]

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How to Set Environment Variables
by 罗勇刚(Yonggang Luo) March 30, 2011

March 30, 2011

http://code.activestate.com/recipes/159462-how-to-set-environment-variables/ Writes environment variables using a batch file wrapper. Overcomes an operating system limitation. *setvar.bat* *----------* *@echo off* *python setvarp.py %1 %2 %3 %4 %5 %6 %7 %8 %9* *settmp* *del settmp.bat* * * *setvarp.py* *----------* *import sys, time, math* *key = sys.argv[1]* *value = eval(' '.join(sys.argv[2:]))* *command = 'set %s=%s\n' % (key, value)* *open('settmp.bat', 'w').write(command)* * * * * *… [View More]

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def-from
by Eric Snow March 29, 2011

March 29, 2011

During the discussion about assignment decorators Nick brought up the idea of adding a def-from syntax: def <NAME> from <BUILDER>(<ARGS>): ... which would effectively call <BUILDER>(<NAME>, <ARGS>, <BODY>) This is much like how meta-classes work, though this would be with functions (sort of meta-functions). Ultimately, they would amount to the same thing. The current "def" statement would just have an implicit builder. During the sprints I … [View More]

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Adding function checks to regex
by MRAB March 28, 2011

March 28, 2011

Some of those who are relative new to regexes sometimes ask how to write a regex which checks that a number is in a range or is a valid date. Although this may be possible, it certainly isn't easy. From what I've read, Perl has a way of including code in a regex, but I don't think that's a good idea However, it occurs to me that there may be a case for being able to call a supplied function to perform such checking. Borrowing some syntax from Perl, it could look like this: def … [View More]

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Python package file type
by Yuval Greenfield March 28, 2011

March 28, 2011

Most of pypi is source distributions (zip/tgz), some of it is eggs, some are msi/exe windows installers. When I started with python I found it a bit confusing, which distribution do I need? "setup.py install" isn't obvious. Do you need an installer for each and every OS? Would it be nice if all of python packages behaved a bit more like firefox extensions (just as an example). Let's say all the packages are still zip/tgz files but with the extension "pypack". Whenever python 4 (a future … [View More]

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Extending error handling on with statements.
by Jakob Bowyer March 28, 2011

March 28, 2011

I personally love using with statements when handling file like objects. This is all well and good until an exception is thrown from the with statement. This is ok if you expect the exception because you can use try and except but personally I feel that another condition to with would feel more 'pythonic' this means that you could fail the with statement with an exception jump to the clause, then jump back to the with statement trying the code in the clause e.g. rather than try: with open('… [View More]

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Two small functional-style-related improvements
by Jan Kaliszewski March 27, 2011

March 27, 2011

Hello. IMHO it'd be nice... 1. ...to add: * operator.is_none -- equivalent to (lambda x: x is None)) * operator.is_not_none -- equivalent tolambda x: x is not None)) ...making using 'is None'/'is not None' tests with any(), all(), filter(), itertools.takewhile/dropwhile() more convenient and readable (possibly also optimised for speed). 2. ...to add: * operator.anti_caller (or e.g. functools.negator?) -- equivalent to: def anti_caller(func): def call_and_negate(*args, **kwargs): … [View More]

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Re: [Python-ideas] [Python-Dev] Dict access with double-dot (syntactic sugar)
by Josiah Carlson March 25, 2011

March 25, 2011

On Fri, Mar 25, 2011 at 9:00 AM, Jameson Quinn <jameson.quinn(a)gmail.com> wrote: > I realized that python already has a way to access the string-based members > of a dict without using quotes: > def expect_a_chair(chair, **kw): > print "Thanks. That chair is %s." % chair > if kw: > for key, val in kw.iteritems(): > print "I wasn't expecting the (%s) %s!" % (val, key) > d = json.loads('{"chair":"… [View More]

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Re: [Python-ideas] Dict access with double-dot (syntactic sugar)
by Jameson Quinn March 25, 2011

March 25, 2011

I realized that python already has a way to access the string-based members of a dict without using quotes: def expect_a_chair(chair, **kw): print "Thanks. That chair is %s." % chair if kw: for key, val in kw.iteritems(): print "I wasn't expecting the (%s) %s!" % (val, key) d = json.loads('{"chair":"comfy","inquisition":"Spanish"}') expect_a_chair(**d) try: expect_a_chair({}) except TypeError: print "No chair." The ** "operator" does this. Notice that nowhere in that python … [View More]

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Different interface for namedtuple?
by Carl M. Johnson March 25, 2011

March 25, 2011

I've been following the discussion on the list about decorators for assignments and whatnot, and while I do think it would be interesting if there were some way to abstract what happens in the creation of classes and defs into a more generalizable structure, for the immediate use case of NamedTuple, wouldn't an interface something like this solve the DRY problem? -- class Point(NewNamedTuple): x, y This interface couldn't have worked in Python 2.6 when namedtuple was introduced, but using the … [View More]

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