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Read Me
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<h1>Python for .NET</h1>
<ul>
<li><a href="#installation">Installation</a></li>
<li><a href="#getting_started">Getting Started</a></li>
<li><a href="#importing">Importing Modules</a></li>
<li><a href="#classes">Using Classes</a></li>
<li><a href="#fields">Fields and Properties</a></li>
<li><a href="#indexers">Using Indexers</a></li>
<li><a href="#methods">Using Methods</a></li>
<li><a href="#delegates">Delegates and Events</a></li>
<li><a href="#exceptions">Exception Handling</a></li>
<li><a href="#arrays">Using Arrays</a></li>
<li><a href="#collections">Using Collections</a></li>
<li><a href="#com">COM Components</a></li>
<li><a href="#types">Type Conversion</a></li>
<li><a href="#assemblies">Using Assemblies</a></li>
<li><a href="#embedding">Embedding Python</a></li>
<li><a href="#license">License</a></li>
</ul>
</td>
<td align="left" valign="top">
<p>
Python for .NET is a package that gives Python programmers nearly seamless
integration with the .NET Common Language Runtime (CLR) and provides a
powerful application scripting tool for .NET developers. Using this package
you can script .NET applications or build entire applications in Python,
using .NET services and components written in any language that targets the
CLR (Managed C++, C#, VB, JScript).
</p>
<p>
Note that this package does <em>not</em> implement Python as a first-class
CLR language - it does not produce managed code (IL) from Python code.
Rather, it is an integration of the C Python engine with the .NET runtime.
This approach allows you to use use CLR services and continue to use existing
Python code and C-based extensions while maintaining native execution speeds
for Python code. If you are interested in a pure managed-code implementation
of the Python language, you should check out the
<a href="http://www.ironpython.com">IronPython</a> project, which is in
active development.
</p>
<p>
Python for .NET is currently compatible with Python releases 2.3 and greater.
Current releases are available at the
<a href="http://pythonnet.sourceforge.net/">
Python for .NET website
</a>. To subscribe to the
<a href="http://mail.python.org/mailman/listinfo/pythondotnet">
Python for .NET mailing list
</a> or read the
<a href="http://mail.python.org/pipermail/pythondotnet/">
online archives
</a> of the list, see the
<a href="http://mail.python.org/mailman/listinfo/pythondotnet">
mailing list information
</a> page.
</p>
<a name="#installation"></a>
<h2>Installation</h2>
<p>
Python for .NET is available as a source release and as a Windows installer
for various versions of Python and the common language runtime from the
<a href="http://pythonnet.sourceforge.net/">
Python for .NET website
</a>. On Windows platforms, you can choose to install .NET-awareness into
an existing Python installation as well as install Python for .NET as a
standalone package.
</p>
<p>
The source release is a self-contained "private" assembly.
Just unzip the package wherever you want it, cd to that directory and run
python.exe to start using it. Note that the source release does not
include a copy of the CPython runtime, so you will need to have installed
Python on your machine before using the source release.
</p>
<p>
<strong>Running on Linux/Mono:</strong> preliminary testing shows that
PythonNet will run under <a href="http://www.go-mono.com">Mono</a>, though
the Mono runtime is not yet complete so there still may be problems. The
Python for .NET integration layer is 100% managed code, so there should be
no long-term issues under Mono - it should work better and better as the
Mono platform matures.
</p>
<p>
It is not currently possible to *build* PythonNet using only the Mono
tools, due to an issue involving the Mono assembler / disassembler. You
should, however, be able to try the pre-built assembly under Mono (or
compile it yourself with the MS tools and run it under Mono).
</p>
<p>
Note that if you are running under Mono on a *nix system, you will need
to have a compatible version of Python installed. You will also need
to create a symbolic link to the copy of libpython2.x.so (in your existing
Python installation) in the PythonNet directory. This is needed to ensure
that the mono interop dll loader will find it by name. For example:
</p>
<pre>
ln -s /usr/lib/libpython2.4.so ./python24.so
</pre>
<a name="getting_started"></a>
<h2>Getting Started</h2>
<p>
A key goal for this project has been that Python for .NET should "work
just the way you'd expect in Python", except for cases that are .NET
specific (in which case the goal is to work "just the way you'd expect
in C#").
</p>
<p>
If you already know Python, you can probably finish this readme and then
refer to .NET docs to figure out anything you need to do. Conversely if
you are familiar with C# or another .NET language, you probably just
need to pick up one of the many good Python books or read the Python
tutorial online to get started.
</p>
<p>
A good way to start is to run <strong>python.exe</strong> and follow along
with the examples in this document. If you get stuck, there are also a
number of demos and unit tests located in the source directory of the
distribution that can be helpful as examples.
</p>
<a name="importing"></a>
<h2>Importing Modules</h2>
<p>
Python for .NET allows CLR namespaces to be treated essentially as
Python packages. The top-level package is named <code>CLR</code>, and
acts as the root for accessing all CLR namespaces:
</p>
<pre>
from CLR.System import String
import CLR.System as System
</pre>
<p>
Types from any loaded assembly may be imported and used in this manner.
The import hook uses "implicit loading" to support automatic loading
of assemblies whose names correspond to an imported namespace:
</p>
<pre>
# This will implicitly load the System.Windows.Forms assembly
from CLR.System.Windows.Forms import Form
</pre>
<p>
Python for .NET uses the PYTHONPATH (sys.path) to look for assemblies
to load, in addition to the usual application base and the GAC. To
ensure that you can implicitly import an assembly, put the directory
containing the assembly in <code>sys.path</code>.
</p>
<p>
To load assemblies with names that do not correspond with a namespace,
you can use the standard mechanisms provided by the CLR:
</p>
<pre>
from CLR.System.Reflection import Assembly
a = Assembly.LoadWithPartialName("SomeAssembly")
# now we can import namespaces defined in that assembly
from CLR.SomeNamespace import Something
</pre>
<p>
Note that CLR modules are "lazy". Because a namespace can contain a
potentially very large number of classes, reflected CLR classes are
created on-demand when they are requested of a CLR module. This means
that using the Python <code>dir()</code> function in the interactive
interpreter will not necessarily show all of the classes available from
a given CLR module (it will only show any classes that have been referenced
to that point in time).
</p>
<p>
It also means that <code>from somemodule import *</code> will not work as
expected. It is possible that it could be made to work in the future,
but for now it would impose a big performance hit and a lot of
complexity to support something that is used relatively rarely and
often considered bad form in Python anyway ;)
</p>
<a name="classes"></a>
<h2>Using Classes</h2>
<p>
Python for .NET allows you to use any non-private classes, structs,
interfaces, enums or delegates from Python. To create an instance of
a managed class, you use the standard instantiation syntax, passing
a set of arguments that match one of its public constructors:
</p>
<pre>
from CLR.System.Drawing import Point
p = Point(5, 5)
</pre>
<p>
You can also subclass managed classes in Python. See the
<code>helloform.py</code> file in the <code>/demo</code> directory of the
distribution for a simple Windows Forms example that demonstrates
subclassing a managed class.
</p>
<a name="fields"></a>
<h2>Fields And Properties</h2>
<p>
You can get and set fields and properties of CLR objects just as if
they were regular attributes:
</p>
<pre>
from CLR.System import Environment
name = Environment.MachineName
Environment.ExitCode = 1
</pre>
<a name="indexers"></a>
<h2>Using Indexers</h2>
<p>
If a managed object implements one or more indexers, you can call
the indexer using standard Python indexing syntax:
</p>
<pre>
from CLR.System.Collections import Hashtable
table = Hashtable()
table["key 1"] = "value 1"
</pre>
<p>
Overloaded indexers are supported, using the same notation one
would use in C#:
</p>
<pre>
items[0, 2]
items[0, 2, 3]
</pre>
<a name="methods"></a>
<h2>Using Methods</h2>
<p>
Methods of CLR objects behave generally like normal Python methods.
Static methods may be called either through the class or through an
instance of the class. All public and protected methods of CLR objects
are accessible to Python:
</p>
<pre>
from CLR.System import Environment
drives = Environment.GetLogicalDrives()
</pre>
<p>
It is also possible to call managed methods <code>unbound</code> (passing the
instance as the first argument) just as with Python methods. This is
most often used to explicitly call methods of a base class.
</p>
<p>
<em>Note that there is one caveat related to calling unbound methods: it
is possible for a managed class to declare a static method and an
instance method with the same name. Since it is not possible for the
runtime to know the intent when such a method is called unbound, the
static method will always be called.</em>
</p>
<p>
The docstring of CLR a method (__doc__) can be used to view the
signature of the method, including overloads if the CLR method is
overloaded. You can also use the Python <code>help</code> method to inspect
a managed class:
</p>
<pre>
from CLR.System import Environment
print Environment.GetFolderPath.__doc__
help(Environment)
</pre>
<a name="delegates"></a>
<h2>Delegates And Events</h2>
<p>
Delegates defined in managed code can be implemented in Python. A
delegate type can be instantiated and passed a callable Python object
to get a delegate instance. The resulting delegate instance is a true
managed delegate that will invoke the given Python callable when it
is called:
</p>
<pre>
def my_handler(source, args):
print 'my_handler called!'
# instantiate a delegate
d = AssemblyLoadEventHandler(my_handler)
# use it as an event handler
AppDomain.CurrentDomain.AssemblyLoad += d
</pre>
<p>
Multicast delegates can be implemented by adding more callable objects
to a delegate instance:
</p>
<pre>
d += self.method1
d += self.method2
d()
</pre>
<p>
Events are treated as first-class objects in Python, and behave in
many ways like methods. Python callbacks can be registered with event
attributes, and an event can be called to fire the event.
</p>
<p>
Note that events support a convenience spelling similar to that used
in C#. You do not need to pass an explicitly instantiated delegate
instance to an event (though you can if you want). Events support the
<code>+=</code> and <code>-=</code> operators in a way very similar to
the C# idiom:
</p>
<pre>
def handler(source, args):
print 'my_handler called!'
# register event handler
object.SomeEvent += handler
# unregister event handler
object.SomeEvent -= handler
# fire the event
result = object.SomeEvent(...)
</pre>
<a name="exceptions"></a>
<h2>Exception Handling</h2>
<p>
You can raise and catch managed exceptions just the same as you would
pure-Python exceptions:
<pre>
from CLR.System import NullReferenceException
try:
raise NullReferenceException("aiieee!")
except NullReferenceException, e:
print e.Message
print e.Source
</pre>
</p>
<a name="arrays"></a>
<h2>Using Arrays</h2>
<p>
Managed arrays support the standard Python sequence protocols:
</p>
<pre>
items = SomeObject.GetArray()
# Get first item
v = items[0]
items[0] = v
# Get last item
v = items[-1]
items[-1] = v
# Get length
l = len(items)
# Containment test
test = v in items
</pre>
<p>
Multidimensional arrays support indexing using the same notation one
would use in C#:
</p>
<pre>
items[0, 2]
items[0, 2, 3]
</pre>
<a name="collections"></a>
<h2>Using Collections</h2>
<p>
Managed arrays and managed objects that implement the IEnumerable
interface can be iterated over using the standard iteration Python
idioms:
</p>
<pre>
domain = System.AppDomain.CurrentDomain
for item in domain.GetAssemblies():
name = item.GetName()
</pre>
<a name="com"></a>
<h2>Using COM Components</h2>
<p>
Using Microsoft-provided tools such as <strong>aximp.exe</strong> and
<strong>tlbimp.exe</strong>, it is possible to generate managed wrappers
for COM libraries. After generating such a wrapper, you can use the
libraries from Python just like any other managed code.
</p>
<p>
Note: currently you need to put the generated wrappers in the GAC,
in the PythonNet assembly directory or on the PYTHONPATH in order
to load them.
</p>
<a name="types"></a>
<h2>Type Conversion</h2>
<p>
Type conversion under Python for .NET is fairly straightforward - most
elemental Python types (string, int, long, etc.) convert automatically
to compatible managed equivalents (String, Int32, etc.) and vice-versa.
Note that all strings returned from the CLR are returned as unicode.
</p>
<p>
Types that do not have a logical equivalent in Python are exposed as
instances of managed classes or structs (System.Decimal is an example).
</p>
<p>
The .NET architecture makes a distinction between <code>value types</code>
and <code>reference types</code>. Reference types are allocated on the heap,
and value types are allocated either on the stack or in-line within an
object.
</p>
<p>
A process called <code>boxing</code> is used in .NET to allow code to treat
a value type as if it were a reference type. Boxing causes a separate
copy of the value type object to be created on the heap, which then
has reference type semantics.
</p>
<p>
Understanding boxing and the distinction between value types and
reference types can be important when using Python for .NET because
the Python language has no value type semantics or syntax - in
Python "everything is a reference".
</p>
<p>
Here is a simple example that demonstrates an issue. If you are an
experienced C# programmer, you might write the following code:
</p>
<pre>
items = CLR.System.Array.CreateInstance(Point, 3)
for i in range(3):
items[i] = Point(0, 0)
items[0].X = 1 # won't work!!
</pre>
<p>
While the spelling of <code>items[0].X = 1</code> is the same in C# and
Python, there is an important and subtle semantic difference. In C# (and other
compiled-to-IL languages), the compiler knows that Point is a value
type and can do the Right Thing here, changing the value in place.
</p>
<p>
In Python however, "everything's a reference", and there is really no
spelling or semantic to allow it to do the right thing dynamically. The
specific reason that <code>items[0]</code> itself doesn't change is that
when you say <code>items[0]</code>, that getitem operation creates a Python
object that holds a reference to the object at <code>items[0]</code> via a
GCHandle. That causes a ValueType (like Point) to be boxed, so the following
setattr (<code>.X = 1</code>) <em>changes the state of the boxed value, not
the original unboxed value</em>.
</p>
<p>
The rule in Python is essentially: "the result of any attribute or
item access is a boxed value", and that can be important in how you
approach your code.
</p>
<p>
Because there are no value type semantics or syntax in Python, you
may need to modify your approach. To revisit the previous example,
we can ensure that the changes we want to make to an array item
aren't "lost" by resetting an array member after making changes
to it:
</p>
<pre>
items = CLR.System.Array.CreateInstance(Point, 3)
for i in range(3):
items[i] = Point(0, 0)
# This _will_ work. We get 'item' as a boxed copy of the Point
# object actually stored in the array. After making our changes
# we re-set the array item to update the bits in the array.
item = items[0]
item.X = 1
items[0] = item
</pre>
<p>
This is not unlike some of the cases you can find in C# where you have
to know about boxing behavior to avoid similar kinds of <code>lost
update</code> problems (generally because an implicit boxing happened that
was not taken into account in the code).
</p>
<p>
This is the same thing, just the manifestation is a little different
in Python. See the .NET documentation for more details on boxing and
the differences between value types and reference types.
</p>
<a name="assemblies"></a>
<h2>Using Assemblies</h2>
<p>
The Python for .NET runtime uses Assembly.LoadWithPartialName to do
name-based imports, which will usually load the most recent version of
an assembly that it can find.
</p>
<p>
The CLR's ability to load different versions of assemblies side-by-side
is one of the (relatively few) places where the matching of meta-models
between Python and the CLR breaks down (though it is unclear how often
this happens in practice).
</p>
<p>
Because Python import is name-based and unaware of any concept of
versioned modules, the design goal has been for name-based implicit assembly
loading to do something consistent and reasonable (i.e. load most
recent available based on the name).
</p>
<p>
It <em>is</em> possible to load a specific version of an assembly if you
need to using code similar to the following:
</p>
<pre>
from CLR.System.Reflection import Assembly, AssemblyName
name = AssemblyName(...) # set required version, etc.
assembly = Assembly.Load(name)
# now import namespaces from the loaded assembly
</pre>
<p>
Things get a lot more complicated if you need to load more than one
version of a particular assembly (or more likely, you have a dependency
on some library the does so). In this case, the names you access via
the CLR modules will always come from the first version of the
assembly loaded (which will always win in the internals of the Python
for .NET runtime).
</p>
<p>
You <em>can</em> still use particular versions of objects in this case - you
just have to do more work to get the right versions of objects:
</p>
<pre>
from CLR.System.Reflection import Assembly, AssemblyName
from System import Activator
name = AssemblyName(...) # get the right version
assembly = Assembly.Load(name)
type = assembly.GetType("QualifiedNameOf.TheTypeINeed")
obj = Activator.CreateInstance(type)
</pre>
</p>
<a name="embedding"></a>
<h2>Embedding Python</h2>
<p>
<strong>Note:</strong> because Python code running under Python for .NET
is inherently unverifiable, it runs totally under the radar of the security
infrastructure of the CLR so you should restrict use of the Python assembly
to trusted code.
</p>
<p>
The Python runtime assembly defines a number of public classes that
provide a subset of the functionality provided by the Python C API.
</p>
<p>
These classes include PyObject, PyList, PyDict, etc. The source and
the unit tests are currently the only API documentation.. The rhythym
is very similar to using Python C++ wrapper solutions such as CXX.
</p>
<p>
At a very high level, to embed Python in your application you
will need to:
</p>
<ul>
<li>Reference Python.Runtime.dll in your build environment</li>
<li>Call PythonEngine.Intialize() to initialize Python</li>
<li>Call PythonEngine.ImportModule(name) to import a module</li>
</ul>
<p>
The module you import can either start working with your managed app
environment at the time its imported, or you can explicitly lookup and
call objects in a module you import.
</p>
<p>
For general-purpose information on embedding Python in applications, use
www.python.org or Google to find (C) examples. Because Python for .NET is
so closely integrated with the managed environment, you will generally be
better off importing a module and deferring to Python code as early as
possible rather than writing a lot of managed embedding code.
</p>
<p>
<strong>Important Note for embedders:</strong> Python is not free-threaded
and uses a global interpreter lock to allow multi-threaded applications to
interact safely with the Python interpreter. Much more information about
this is available in the Python C API documentation on the www.python.org
Website.
</p>
<p>
When embedding Python in a managed application, you have to manage the GIL
in just the same way you would when embedding Python in a C or C++
application.
</p>
<p>
Before interacting with any of the objects or APIs provided by the
Python.Runtime namespace, calling code must have acquired the Python
global interpreter lock by calling the <code>PythonEngine.AcquireLock</code>
method. The only exception to this rule is the
<code>PythonEngine.Initialize</code> method, which may be called at startup
without having acquired the GIL.
</p>
<p>
When finished using Python APIs, managed code must call a corresponding
<code>PythonEngine.ReleaseLock</code> to release the GIL and allow other
threads to use Python.
</p>
<p>
The AcquireLock and ReleaseLock methods are thin wrappers over the
unmanaged <code>PyGILState_Ensure</code> and <code>PyGILState_Release</code>
functions from the Python API, and the documentation for those APIs applies
to the managed versions.
</p>
<a name="license" />
<h2>License</h2>
<p>
Python for .NET is released under the open source Zope Public License (ZPL).
A copy of the ZPL is included in the distribution, or you can find a copy
of the
<a href="http://pythonnet.sourceforge.net/license.txt">
ZPL online
</a>. Some distributions of this package include a copy of the C Python
dlls and standard library, which are covered by the
<a href="http://www.python.org/license.html">
Python license
</a>.
</p>
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