My first Python program
Hallvard B Furuseth
h.b.furuseth at usit.uio.no
Thu Oct 14 15:07:21 CEST 2010
>> You can't really rely on the destructor __del__ being called.
> Interesting. Do I just rely on files getting closed?
Sometimes, but that's not it. Think Lisp, not C++. __del__ is not that
useful. Python is garbage-collected and variables have dynamic lifetime,
so the class cannot expect __del__ to be called in a timely manner.
Destructors have several issues, see __del__ in the Python reference.
A class which holds an OS resource like a file, should provide a context
manager and/or a release function, the latter usually called in a
'finally:' block. When the caller doesn't bother with either, the class
often might as well depend on the destructor in 'file'.
Still, open().read() is common. open().write() is not. The C
implementation of Python is reference-counted on top of GC, so the file
is closed immediately. But this way, exceptions from close() are lost.
Python cannot propagate them up the possibly-unrelated call chain.
Some other points:
For long strings, another option is triple-quoting as you've seen in doc
strings: print """foo
def emit(self, template, func = None):
# hey, at least it's not a global variable, amirite?
SourceFile.copyright = copyright_file.read()
emit() can use self.copyright instead of SourceFile.copyright.
I've written such code, but I suppose the proper way is to use a
classmethod to set it, so you can see in the class how the copyright
gets there. SourceFile.<classmethod>() and self.<classmethod>() both
get called with the class as 1st argument.
def setup_copyright(cls, fname):
cls.copyright = open(fname).read()
setup_copyright = classmethod(setup_copyright)
# In python >= 2.4 you can instead say @classmethod above the def.
SourceFile.__repr__() looks like it should be a __str__(). I haven't
looked at how you use it though. But __repr__ is supposed to
look like a Python expression to create the instance: repr() = '',
or a generic '<foo instance>': repr(id) = '<built-in function id>'.
"How new are list comprehensions?"
Python 2.0, found as follows:
- Google python list comprehensions.
- Check the PEP (Python Enhancement Proposal) which shows up. PEPs
are the formal documents for info to the community, for the Python
development process, etc. <http://www.python.org/dev/peps/pep-0202/>:
Title: List Comprehensions
Type: Standards Track
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