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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="#generics">Using Generics</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="#genericmethods">Overloaded and Generic 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="#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> 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#"). In addition, with the IronPython project
gaining traction, it is my goal that code written for IronPython
run without modification under Python for .NET.
</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>
<p> Note that if you have installed CLR support into your existing
Python installation (rather than using the included python.exe),
you will need to use the line: "'import clr" (lower-case!) to
initially load the clr extension module before trying the
following examples.
</p>
<a name="importing"></a>
<h2>Importing Modules</h2>
<p> Python for .NET allows CLR namespaces to be treated essentially
as Python packages. </p>
<p>
</p>
<pre> from System import String
from System.Collections import *
</pre>
<p>
<em> Note that earlier releases of Python for .NET required you to
import modules through a special top-level package named <code>CLR</code>.
This is no longer required if you are starting python from the
managed python.exe from this distribution.<br>
<code>CLR</code> has been deprecated in favor of the more
pythonic <code>clr</code>, though the syntax is still supported
for backward compatibility.
</em>
</p>
<p> Types from any loaded assembly may be imported and used in this
manner. To load an assembly, use the "AddReference" function in
the "clr" module:
</p>
<pre>
import clr
clr.AddReference("System.Windows.Forms")
from System.Windows.Forms import Form
</pre>
<p>
<em> Note that earlier releases of Python for .NET relied on
"implicit loading" to support automatic loading of assemblies
whose names corresponded to an imported namespace. Implicit
loading still works for backward compatibility, but will be
removed in a future release so it is recommended to use the
clr.AddReference method.
</em>
</p>
<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>
<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 System.Drawing import Point
p = Point(5, 5)
</pre>
<p> In most cases, Python for .NET can determine the correct
constructor to call automatically based on the arguments. In some
cases, it may be necessary to call a particular overloaded
constructor, which is supported by a special "__overloads__"
attribute, which will soon be deprecated in favor of iPy
compatible "Overloads", on a class:
</p>
<pre> from System import String, Char, Int32
s = String.Overloads[Char, Int32]('A', 10)
s = String.__overloads__[Char, Int32]('A', 10)
</pre>
<a name="generics"></a>
<h2>Using Generics</h2>
<p> When running under versions of the .NET runtime greater than
2.0, you can use generic types. A generic type must be bound to
create a concrete type before it can be instantiated. Generic
types support the subscript syntax to create bound types:
</p>
<pre> from System.Collections.Generic import Dictionary
from System import *
dict1 = Dictionary[String, String]()
dict2 = Dictionary[String, Int32]()
dict3 = Dictionary[String, Type]()
</pre>
<p> When you pass a list of types using the subscript syntax, you
can also pass a subset of Python types that directly correspond to
.NET types:
</p>
<pre> dict1 = Dictionary[str, str]()
dict2 = Dictionary[str, int]()
dict3 = Dictionary[str, Decimal]()
</pre>
<p> This shorthand also works when explicitly selecting generic
methods or specific versions of overloaded methods and
constructors (explained later).
</p>
<p> You can also subclass managed classes in Python, though members
of the Python subclass are not visible to .NET code. 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 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 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 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 System import Environment
print Environment.GetFolderPath.__doc__
help(Environment)
</pre>
<a name="genericmethods"></a>
<h2>Overloaded and Generic Methods</h2>
<p> While Python for .NET will generally be able to figure out the
right version of an overloaded method to call automatically, there
are cases where it is desirable to select a particular method
overload explicitly.
</p>
<p> Methods of CLR objects have an "__overloads__", which will soon
be deprecated in favor of iPy compatible Overloads, attribute that
can be used for this purpose :
</p>
<pre> from System import Console
Console.WriteLine.Overloads[bool](true)
Console.WriteLine.Overloads[str]("true")
Console.WriteLine.__overloads__[int](42)
</pre>
<p> Similarly, generic methods may be bound at runtime using the
subscript syntax directly on the method:
</p>
<pre> someobject.SomeGenericMethod[int](10)
someobject.SomeGenericMethod[str]("10")
</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:
</p>
<pre> from System import NullReferenceException
try:
raise NullReferenceException("aiieee!")
except NullReferenceException, e:
print e.Message
print e.Source
</pre>
<p></p>
<a name="arrays"></a>
<h2>Using Arrays</h2>
<p> The type <code>System.Array</code> supports the subscript
syntax in order to make it easy to create managed arrays from
Python:
</p>
<pre> from System import Array
myarray = Array[int](10)
</pre>
<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 = 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 = 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="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>
</a>
<p><a name="license"> 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><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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