How to use SQLAlchemy & MySQL with py2exe

py2exe doesn't automatically detect the dependencies, so you have to explicitly tell it to include the modules in the setup function.

E.g.

from distutils.core import setup
import py2exe

setup(
    console=['myprog.py'],
    options=dict(
        py2exe=dict(
            includes=['sqlalchemy.databases.mysql', 'MySQLdb']
        )
    )
)

How to compile the Apache QPID C++ broker from SVN without checking out the Java broker.

I was trying to check out and build the Apache Qpid C++ broker from source. The instructions say to check out  http://svn.apache.org/repos/asf/qpid/branches/0.6-release/qpid/.  But that Java broker dir is HUGE and I don't event WANT the Java broker!

So at first I just checked out the cpp dir. Why not? Well the build breaks in a strange way, something about cannot execute rubygen.mk.

make[1]: execvp: ./rubygen.mk: Permission denied 

It turns out that there is a dependency on the specs dir. But autoconf doesn't tell you about it. So you must check out BOTH dirs to a common dir, then build the cpp broker.

eg:

mkdir qpid

cd qpid

svn co http://svn.apache.org/repos/asf/qpid/branches/0.6-release/qpid/cpp cpp

svn co http://svn.apache.org/repos/asf/qpid/branches/0.6-release/qpid/specs specs

cd cpp

./bootstrap

./configure

make

make check

make install

That should do it.

SWIG, Python, and stopping C++ objects from being destroyed when they fall out of scope.

I recently had an issue with SWIG and Python. I had wrapped two C++ classes, lets call them class A and class B. Class B takes as an argument to it's constructor a pointer to an instance of class A and saves it in a member variable for later use.

EG C++ code:

class A {
    void;
};

class B {
  public:
    B(A* a) { this->a = a; }
    A* a;
};

Now in Python I do this:

def make_B():
    a = A()
    b = B(a)
    return b

b = make_B()
print b.a # garbage, or segfault

The problem is that Python doesn't know that b is holding a reference to a, and it destroys a when it falls out of scope.

I spent hours digging through the SWIG documentation trying to find an elegant way to inform SWIG that it should increment the reference count on a when I call the b constructor, to no avail. Finally I came up with a hack that works pretty well. I used the %feature("pythonappend") mechanism to extend the B proxy class constructor so that it saves a reference to the a it's passed. The code in the .i file looks like this

%feature("pythonappend") B(A*) %{
    self._ref_to_a = args[0] # a refcount +=1
%}

which modifies the proxy class constructor thus:

  def __init__(self, *args): 
      this = _mymodule.new_B(*args)
      try: self.this.append(this)
      except: self.this = this
+     ref_to_a = args[0] # a refcount +=1

Now when a B is created it's proxy object keeps a reference to the A it's passed, causing python to increment the A's reference count, preventing it from being destroyed when it falls out of scope. When the B proxy object is destroyed, the refcount on A will be decremented, making it available for garbage collection as well.

You can make this a bit more convenient by defining a SWIG macro

%define %SAVE_ARGS_REF(function, argnum)
%feature("pythonappend") function %{
        self.__saved_ref_ ## argnum = args[ argnum ] 
%}
%enddef

%SAVE_ARGS_REF( B(A*), 0)

The only potential issue I can see with this approach is that it clutters the namespace; it's up to the programmer not to clobber anything.

It's also not particularly elegant; it would be nice if SWIG offered a simple typemap or decorator or something to indicate that a particular object saves a reference to another object for the duration of it's life.

Note: I've confirmed this technique works, but I haven't tested the specific code in this post, cut and pasters beware.