359 lines
11 KiB
Markdown
359 lines
11 KiB
Markdown
+++
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title = "A Python Transaction Class"
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date = 2015-10-13T10:20:43+00:00
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[taxonomies]
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tags = ["python", "programming"]
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+++
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This is a repost of a blog post of 2008. Just for the reference :-)
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This class allows sub-classes to commit changes to an instance to a history, and rollback to previous states.
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<!-- more -->
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The final class with an extension for `__setstate__` and `__getstate__` can be found here:
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[transaction.py](/files/transaction.py)
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and [transaction_test.py](/files/transaction_test.py).
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Now to the story, that led to it: I had the need for a transaction class in python and browsing the python cookbook led me to a [small Transaction class](http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/551788).
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```python
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$ python
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Python 2.5.1 (r251:54863, Oct 30 2007, 13:45:26)
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[GCC 4.1.2 20070925 (Red Hat 4.1.2-33)] on linux2
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Type "help", "copyright", "credits" or "license" for more information.
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>>> class Transaction(object):
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... def __init__(self):
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... self.log = []
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... def commit(self):
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... self.log.append(self.__dict__.copy())
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... def rollback(self):
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... try:
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... self.__dict__.update(self.log.pop(-1))
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... except IndexError:
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... pass
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...
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```
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Ok, lets have some fun with it.
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```python
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>>> class A(Transaction):
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... pass
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...
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>>> a = A()
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>>> a.test = True
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>>> a.commit()
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>>> a.test = False
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>>> a
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'self.__dict__ = {'test': False, 'log': [{'test': True, 'log': [...]}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': True, 'log': []}'
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```
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Nice. Let's see if we can commit and rollback several times.
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```python
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>>> a = A()
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>>> a.test = 1
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>>> a.commit()
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>>> a.test = 2
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>>> a.commit()
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>>> a.test = 3
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>>> a
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'self.__dict__ = {'test': 3, 'log': [{'test': 1, 'log': [...]}, {'test': 2, 'log': [...]}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': 2, 'log': [{'test': 1, 'log': [...]}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': 1, 'log': []}'
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```
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Ok.. works :) Let's try some lists.
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```python
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>>> a = A()
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>>> a.test = [ 0, 1 ]
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>>> a.commit()
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>>> a.test.append(2)
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>>> a
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'self.__dict__ = {'test': [0, 1, 2], 'log': [{'test': [0, 1, 2], 'log': [...]}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': [0, 1, 2], 'log': []}'
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```
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Doh! Ok, someone mentioned that already in the comments. copy.deepcopy() is the key.
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```python
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>>> import copy
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>>>
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>>> class Transaction2(Transaction):
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... def commit(self, **kwargs):
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... self.log.append(copy.deepcopy(self.__dict__))
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...
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>>> class A(Transaction2):
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... pass
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...
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>>> a = A()
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>>> a.test = [ 0, 1 ]
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>>> a.commit()
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>>> a.test.append(2)
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>>> a
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'self.__dict__ = {'test': [0, 1, 2], 'log': [{'test': [0, 1], 'log': []}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': [0, 1], 'log': []}'
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```
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Ah, works. Very good. Now another check:
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```python
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>>> a = A()
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>>> a.test = 1
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>>> a.commit()
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>>> a
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'self.__dict__ = {'test': 1, 'log': [{'test': 1, 'log': []}]}'
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>>> a.other = 2
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>>> a
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'self.__dict__ = {'test': 1, 'other': 2, 'log': [{'test': 1, 'log': []}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': 1, 'other': 2, 'log': []}'
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>>> a.other
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2
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```
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Oh, a leftover... seems like `self.__dict__` has to be cleared, before the update.
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```python
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>>> class Transaction3(Transaction2):
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... def rollback(self, **kwargs):
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... try:
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... state = self.log.pop(-1)
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... self.__dict__.clear()
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... self.__dict__.update(state)
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... except IndexError:
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... pass
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...
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>>> class A(Transaction3):
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... pass
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...
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>>> a = A()
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>>> a.test = 1
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>>> a.commit()
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>>> a
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'self.__dict__ = {'test': 1, 'log': [{'test': 1, 'log': []}]}'
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>>> a.other = 2
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>>> a
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'self.__dict__ = {'test': 1, 'other': 2, 'log': [{'test': 1, 'log': []}]}'
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'test': 1, 'log': []}'
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>>> a.other
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Traceback (most recent call last):
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File "", line 1, in
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AttributeError: 'A' object has no attribute 'other'
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>>>
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```
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Ah, works. Very good. Ok, more tests...
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```python
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>>> b = a.ncls
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>>> a.ncls.test = True
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>>> a.commit()
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>>> a.ncls.test = False
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'ncls': 'self.__dict__ = {'test': True, 'log': []}', 'log': []}'
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>>> b
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'self.__dict__ = {'test': False, 'log': []}'
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```
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Oh, what if we work with "b", which stills holds the old value? Maybe we should commit() all our attributes also, traversing through them all? Ok, here is such a beast:
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```python
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>>> class TransactionNew1(object):
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... def _docommit(self):
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... if "log" not in self.__dict__:
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... self.__dict__["log"] = list()
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...
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... self.__dict__["log"].append(copy.deepcopy(self.__dict__))
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...
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... def _dorollback(self):
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... if "log" not in self.__dict__:
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... return
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... try:
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... state = self.__dict__["log"].pop(-1)
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... self.__dict__.clear()
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... self.__dict__.update(state)
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... except IndexError:
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... pass
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...
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... def commit(self, **kwargs):
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... # commit ourselves, then our childs
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... self._docommit()
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... if kwargs.get("deep", True):
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... for child in self.__dict__.values():
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... if isinstance(child, self.__class__):
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... child.commit()
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...
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... def rollback(self, **kwargs):
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... # rollback our childs, then ourselves
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... if kwargs.get("deep", True):
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... for child in self.__dict__.values():
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... if isinstance(child, self.__class__):
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... child.rollback()
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... self._dorollback()
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...
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... def __repr__(self):
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... return "'self.__dict__ = %s'" % self.__dict__
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...
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>>> class A(TransactionNew1):
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... pass
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...
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>>> a = A()
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>>> a.ncls = A()
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>>> b = a.ncls
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>>> a.ncls.test = True
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>>> a.commit()
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>>> a.ncls.test = False
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>>> a.rollback()
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>>> a
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'self.__dict__ = {'ncls': 'self.__dict__ = {'test': True}', 'log': []}'
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>>> b
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'self.__dict__ = {'test': True, 'log': []}'
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```
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Ok... looks good, but we lost the reference. id(b) != id(a.ncls) ... ( update: this is fixed in the final version ) Working with it revealed also:
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```python
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>>> a = A()
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>>> b = a
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>>> for i in xrange(3):
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... b.n = A()
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... b.t = "test"
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... b = b.n
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... a.commit()
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...
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>>> a
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'self.__dict__ = {'log': [{'n': 'self.__dict__ = {}', 'log': [], 't': 'test'}, {'n': 'self.__dict__ = {'log': [{'log': []}], 't': 'test', 'n': 'self.__dict__ = {}'}', 'log': [{'t': 'test', 'log': [], 'n': 'self.__dict__ = {}'}], 't': 'test'}, {'n': 'self.__dict__ = {'log': [{'log': []}, {'log': [{'log': []}], 't': 'test', 'n': 'self.__dict__ = {}'}], 't': 'test', 'n': 'self.__dict__ = {'log': [{'log': []}], 't': 'test', 'n': 'self.__dict__ = {}'}'}', 'log': [{'t': 'test', 'log': [], 'n': 'self.__dict__ = {}'}, {'t': 'test', 'log': [{'n': 'self.__dict__ = {}', 't': 'test', 'log': []}], 'n': 'self.__dict__ = {'t': 'test', 'log': [{'log': []}], 'n': 'self.__dict__ = {}'}'}], 't': 'test'}], 't': 'test', 'n': 'self.__dict__ = {'t': 'test', 'log': [{'log': []}, {'n': 'self.__dict__ = {}', 't': 'test', 'log': [{'log': []}]}, {'n': 'self.__dict__ = {'log': [{'log': []}], 't': 'test', 'n': 'self.__dict__ = {}'}', 't': 'test', 'log': [{'log': []}, {'log': [{'log': []}], 't': 'test', 'n': 'self.__dict__ = {}'}]}], 'n': 'self.__dict__ = {'t': 'test', 'log': [{'log': []}, {'n': 'self.__dict__ = {}', 't': 'test', 'log': [{'log': []}]}], 'n': 'self.__dict__ = {'log': [{'log': []}]}'}'}'}'
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>>> len(str(a))
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1192
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```
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Hmm... seems strange.. Ah, self.log was also copied with copy.deepcopy(). So, we have multiple useless copies. Let's "pop()" the state from self.__dict__ before the deepcopy.
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```python
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>>> class TransactionNew2(TransactionNew1):
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... def _docommit(self):
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... if "log" in self.__dict__:
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... oldstate = self.__dict__.pop("log")
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... else:
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... oldstate = None
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... state = copy.deepcopy(self.__dict__)
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... if oldstate:
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... state["log"] = oldstate
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... self.__dict__["log"] = state
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... def _dorollback(self):
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... if "log" not in self.__dict__:
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... return
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... try:
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... state = self.__dict__["log"]
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... self.__dict__.clear()
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... self.__dict__.update(state)
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... except IndexError:
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... pass
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...
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>>> class A(TransactionNew2):
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... pass
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...
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>>>
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>>> a = A()
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>>> b = a
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>>> for i in xrange(3):
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... b.n = A()
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... b.t = "test"
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... b = b.n
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... a.commit()
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...
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>>> a
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'self.__dict__ = {'log': {'log': {'log': {'t': 'test', 'n': 'self.__dict__ = {}'}, 't': 'test', 'n': 'self.__dict__ = {'log': {}, 't': 'test', 'n': 'self.__dict__ = {}'}'}, 't': 'test', 'n': 'self.__dict__ = {'log': {'t': 'test', 'n': 'self.__dict__ = {}'}, 't': 'test', 'n': 'self.__dict__ = {'log': {}, 't': 'test', 'n': 'self.__dict__ = {}'}'}'}, 't': 'test', 'n': 'self.__dict__ = {'t': 'test', 'log': {'log': {'t': 'test', 'n': 'self.__dict__ = {}'}, 't': 'test', 'n': 'self.__dict__ = {'log': {}, 't': 'test', 'n': 'self.__dict__ = {}'}'}, 'n': 'self.__dict__ = {'t': 'test', 'log': {'t': 'test', 'n': 'self.__dict__ = {}'}, 'n': 'self.__dict__ = {'log': {}}'}'}'}'
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>>> len(str(a))
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671
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```
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Ok, saved us a bit of state length. The final version has:
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```python
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>>> a
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'self.__dict__ = {'__l': {'__l': {'__l': {'t': 'test'}, 't': 'test'}, 't': 'test'}, 't': 'test', 'n': 'self.__dict__ = {'__l': {'__l': {'t': 'test'}, 't': 'test'}, 't': 'test', 'n': 'self.__dict__ = {'__l': {'t': 'test'}, 't': 'test', 'n': 'self.__dict__ = {'__l': {}}'}'}'}'
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>>> len(str(a))
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275
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```
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Now another thing:
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```python
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>>> a = A()
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>>> a.n = a
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>>> a.commit()
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File "/usr/lib64/python2.5/copy.py", line 162, in deepcopy
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y = copier(x, memo)
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RuntimeError: maximum recursion depth exceeded
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```
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... Oh, oh! Recursion in commit()... Now we have to check, if we have been there.
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```python
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>>> class TransactionNew3(TransactionNew2):
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... def _checksetseen(self, seen):
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... if id(self) in seen:
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... import sys
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... sys.stderr.write("Recursion detected... \n")
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... return True
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... seen.add(id(self))
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... return False
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...
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... def commit(self, **kwargs): # pylint: disable-msg=W0613
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... seen = kwargs.get("_commit_seen", set())
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... if self._checksetseen(seen):
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... return
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... # commit ourselves, then our childs
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... self._docommit()
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... if kwargs.get("deep", True):
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... for child in self.__dict__.values():
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... if isinstance(child, self.__class__):
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... child.commit(_commit_seen = seen)
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...
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... def rollback(self, **kwargs):
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... seen = kwargs.get("_rollback_seen", set())
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... if self._checksetseen(seen):
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... return
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... # rollback our childs, then ourselves
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... if kwargs.get("deep", True):
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... for child in self.__dict__.values():
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... if isinstance(child, self.__class__):
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... child.rollback(_rollback_seen = seen)
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... self._dorollback()
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...
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>>>
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>>> class A(TransactionNew3):
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... pass
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...
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>>> a = A()
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>>> a.n = a
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>>> a.commit()
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Recursion detected...
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>>>
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```
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The final class with an extension for `__setstate__` and `__getstate__` can be found here: [transaction.py](https://harald.hoyer.xyz/files/transaction.py) and [transaction_test.py](/files/transaction_test.py). Have fun with it :-)
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