tests/test-ctxmanager.py

util: introduce ctxmanager, to avoid nested try/finally blocks

This is similar in spirit to contextlib.nested in Python <= 2.6,
but uses an extra level of indirection to avoid its inability to
clean up if an __enter__ method raises an exception.

Why add this mechanism?  It greatly simplifies scoped resource
management, and lets us eliminate several hundred lines of try/finally
blocks.  In many of these cases the "finally" is separated from the
"try" by hundreds of lines of code, which makes the connection
between resource acquisition and disposal difficult to follow.

(The preferred mechanism would be the "multi-with" syntax of 2.7+,
but Mercurial can't move to 2.7 for a while.)

Intended use:

>>> with ctxmanager(lambda: file('foo'), lambda: file('bar')) as c:
>>>    f1, f2 = c()

This will open both foo and bar when c() is invoked, and will close
both upon exit from the block.  If the attempt to open bar raises
an exception, the block will not be entered - but foo will still
be closed.

from __future__ import absolute_import

import silenttestrunner
import unittest

from mercurial.util import ctxmanager

class contextmanager(object):
    def __init__(self, name, trace):
        self.name = name
        self.entered = False
        self.exited = False
        self.trace = trace

    def __enter__(self):
        self.entered = True
        self.trace(('enter', self.name))
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        self.exited = exc_type, exc_val, exc_tb
        self.trace(('exit', self.name))

    def __repr__(self):
        return '<ctx %r>' % self.name

class ctxerror(Exception):
    pass

class raise_on_enter(contextmanager):
    def __enter__(self):
        self.trace(('raise', self.name))
        raise ctxerror(self.name)

class raise_on_exit(contextmanager):
    def __exit__(self, exc_type, exc_val, exc_tb):
        self.trace(('raise', self.name))
        raise ctxerror(self.name)

def ctxmgr(name, trace):
    return lambda: contextmanager(name, trace)

class test_ctxmanager(unittest.TestCase):
    def test_basics(self):
        trace = []
        addtrace = trace.append
        with ctxmanager(ctxmgr('a', addtrace), ctxmgr('b', addtrace)) as c:
            a, b = c()
            c.atexit(addtrace, ('atexit', 'x'))
            c.atexit(addtrace, ('atexit', 'y'))
        self.assertEqual(trace, [('enter', 'a'), ('enter', 'b'),
                                 ('atexit', 'y'), ('atexit', 'x'),
                                 ('exit', 'b'), ('exit', 'a')])

    def test_raise_on_enter(self):
        trace = []
        addtrace = trace.append
        with self.assertRaises(ctxerror):
            with ctxmanager(ctxmgr('a', addtrace),
                           lambda: raise_on_enter('b', addtrace)) as c:
                c()
                addtrace('unreachable')
        self.assertEqual(trace, [('enter', 'a'), ('raise', 'b'), ('exit', 'a')])

    def test_raise_on_exit(self):
        trace = []
        addtrace = trace.append
        with self.assertRaises(ctxerror):
            with ctxmanager(ctxmgr('a', addtrace),
                           lambda: raise_on_exit('b', addtrace)) as c:
                c()
                addtrace('running')
        self.assertEqual(trace, [('enter', 'a'), ('enter', 'b'), 'running',
                                 ('raise', 'b'), ('exit', 'a')])

if __name__ == '__main__':
    silenttestrunner.main(__name__)