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<html>

<head>

<title>Python for .NET</title>

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</head>

<body>

<tbody>

<tr>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

<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>

</td>

</tr>

</tbody>

</table>

</body>

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