On Thursday, March 1, 2018 at 12:15:57 AM UTC, Paul Rubin wrote:
> RAII is similar to Python's "with" statement. So it sounds like OP
> wants to replace one "malignant carbuncle" with another one.
I would like to understand why you think RAII is not substantially more
pythonic than "With". Below I have sketched out a File-like resource
management scenario using both PEP343 and RAII idioms for comparison.
First lets look at the implementation of the competing resource management
classes:-
class PEP343FileAccess():
'''
A File Access-like resource with self-contained Context Manager for use
with "with".
This could be acheived with two separate classes but I don't think that
adds anything except lines of code.
'''
# Structors
def __init__(self, filename, mode):
self.filename = filename
self.mode = mode
self.handle = None
# dummy file content accessible only when handle is not None
self.lines = ['one', 'two', 'three']
# PEP343 Context Manager compliance
def __enter__(self):
self.handle = low_level_file_open(self.filename, self.mode) # fictitious
return self
def __exit__(self, extype, exvalue, extraceback):
low_level_file_close(self.handle) # fictitious function
self.handle = None
# Example methods
def __iter__(self):
assert self.handle, "File has been closed"
for line in self.lines:
yield line
def write(self, line):
assert self.handle, "File has been closed"
self.lines.append(line)
class RAIIFileAccess():
'''File Access-like Resource using RIAA idiom'''
# Structors
def __init__(self, filename, mode):
self.handle = low_level_file_open(filename, mode) # fictitious
# dummy content accessible as long as the object exists (invariant)
self.lines = ['one', 'two', 'three']
def __del__(self):
low_level_file_close(self.handle) # fictitious function
# Example methods
def __iter__(self):
for line in self.lines:
yield line
def write(self, line):
self.lines.append(line)
What I see is that PEP343 requires two new methods: __enter__ & __exit__.
RIAA requires no new methods.
RIAA resources are invariant: If you have a reference to it you can use it.
PEP343 resources can not be invariant: To be robust the enter/exit state
must be tracked and checked. (assert self.handle in the example)
Now lets look at example resource usage:-
def pep343_example():
with PEP343FileAccess("src.txt", 'r') as src,
PEP343FileAccess("dst.txt", 'w') as dst:
for line in src:
dst.write(line)
def raii_example():
src = RAIIFileAccess("src.txt", 'r')
dst = RAIIFileAccess("dst.txt", 'w')
for line in src:
dst.write(line)
PEP343 requires specialised "with" syntax, RIAA requires no new syntax.
Furthermore, although src & dst objects are still accessible outside the
PEP343 "with" block they are not in a usable state (not invariant).
In the RIAA case the resources are guaranteed to be in a usable state as long
as any reference exists (invariant). References can also be safely
passed around.The resource will be freed/closed when the last man has finished
with it, even in the face of exceptions.
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