Python for .NET

Python for .NET (pythonnet) is a package that gives Python programmers nearly seamless integration with the .NET 4.0+ Common Language Runtime (CLR) on Windows and Mono runtime on Linux and OSX. Python for .NET 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 (C#, VB.NET, F#, C++/CLI).

Note that this package does not 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 CPython engine with the .NET or Mono runtime. This approach allows you to use CLR services and continue to use existing Python code and C-API 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 IronPython project, which is in active development.

Python for .NET is currently compatible and tested with Python releases 2.7, 3.3, 3.4, 3.5, and 3.6. Current releases are available at the Python for .NET website. To subscribe to the Python for .NET mailing list or read the online archives of the list, see the mailing list information page. Use the Python for .NET issue tracker to report issues.

Installation

Python for .NET is available as a source release on GitHub and as a binary wheel distribution for all supported versions of Python and the common language runtime from the Python Package Index.

The source release is a self-contained “private” assembly. Just unzip the package wherever you want it, cd to that directory, build the solution python setup.py build_ext --inplace. Once you start up Python or IPython interpreter in this directory or append this directory to sys.path, then after import clr statement .NET assemblies can be used. You can also run nPython.exe (mono nPython.exe on *nix) to check how python can be embedded in console .NET application. 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.

Running on Linux/Mono: Unit testing shows that PythonNet will run under Mono, though the Mono runtime is less supported so there still may be problems.

Getting Started

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 having established a community, it is my goal that code written for IronPython run without modification under Python for .NET.

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.

A good way to start is to interactively explore .NET usage in python interpreter by following 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.

Importing Modules

Python for .NET allows CLR namespaces to be treated essentially as Python packages.

from System import String
from System.Collections import *

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:

import clr
clr.AddReference("System.Windows.Forms")
from System.Windows.Forms import Form

Note 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.

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 sys.path.

Using Classes

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:

from System.Drawing import Point

p = Point(5, 5)

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:

from System import String, Char, Int32

s = String.Overloads[Char, Int32]('A', 10)
s = String.__overloads[__Char, Int32]('A', 10)

Using Generics

Pythonnet also supports 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:

from System.Collections.Generic import Dictionary
from System import *

dict1 = Dictionary[String, String]()
dict2 = Dictionary[String, Int32]()
dict3 = Dictionary[String, Type]()

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:

dict1 = Dictionary[str, str]()
dict2 = Dictionary[str, int]()
dict3 = Dictionary[str, Decimal]()

This shorthand also works when explicitly selecting generic methods or specific versions of overloaded methods and constructors (explained later).

You can also subclass managed classes in Python, though members of the Python subclass are not visible to .NET code. See the helloform.py file in the /demo directory of the distribution for a simple Windows Forms example that demonstrates subclassing a managed class.

Fields And Properties

You can get and set fields and properties of CLR objects just as if they were regular attributes:

from System import Environment

name = Environment.MachineName
Environment.ExitCode = 1

Using Indexers

If a managed object implements one or more indexers, you can call the indexer using standard Python indexing syntax:

from System.Collections import Hashtable

table = Hashtable()
table["key 1"] = "value 1"

Overloaded indexers are supported, using the same notation one would use in C#:

items[0, 2]

items[0, 2, 3]

Using Methods

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:

from System import Environment

drives = Environment.GetLogicalDrives()

It is also possible to call managed methods unbound (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.

Note 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.

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 help method to inspect a managed class:

from System import Environment

print(Environment.GetFolderPath.__doc__)

help(Environment)

Overloaded and Generic Methods

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.

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:

from System import Console

Console.WriteLine.Overloads[bool](true)
Console.WriteLine.Overloads[str]("true")
Console.WriteLine.__overloads[__int](42)

Similarly, generic methods may be bound at runtime using the subscript syntax directly on the method:

someobject.SomeGenericMethod[int](10)
someobject.SomeGenericMethod[str]("10")

Delegates And Events

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:

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

Multicast delegates can be implemented by adding more callable objects to a delegate instance:

d += self.method1
d += self.method2
d()

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.

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 += and -= operators in a way very similar to the C# idiom:

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(...)

Exception Handling

You can raise and catch managed exceptions just the same as you would pure-Python exceptions:

from System import NullReferenceException

try:
    raise NullReferenceException("aiieee!")
except NullReferenceException as e:
    print(e.Message)
    print(e.Source)

Using Arrays

The type System.Array supports the subscript syntax in order to make it easy to create managed arrays from Python:

from System import Array

myarray = Array[int](10)

Managed arrays support the standard Python sequence protocols:

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

Multidimensional arrays support indexing using the same notation one would use in C#:

items[0, 2]

items[0, 2, 3]

Using Collections

Managed arrays and managed objects that implement the IEnumerable interface can be iterated over using the standard iteration Python idioms:

domain = System.AppDomain.CurrentDomain

for item in domain.GetAssemblies():
    name = item.GetName()

Using COM Components

Using Microsoft-provided tools such as aximp.exe and tlbimp.exe, 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.

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.

Type Conversion

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.

Types that do not have a logical equivalent in Python are exposed as instances of managed classes or structs (System.Decimal is an example).

The .NET architecture makes a distinction between value types and reference types. Reference types are allocated on the heap, and value types are allocated either on the stack or in-line within an object.

A process called boxing 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.

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”.

Here is a simple example that demonstrates an issue. If you are an experienced C# programmer, you might write the following code:

items = System.Array.CreateInstance(Point, 3)
for i in range(3):
    items[i] = Point(0, 0)

items[0].X = 1 # won't work!!

While the spelling of items[0].X = 1 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.

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 items[0] itself doesn’t change is that when you say items[0], that getitem operation creates a Python object that holds a reference to the object at items[0] via a GCHandle. That causes a ValueType (like Point) to be boxed, so the following setattr (.X = 1) changes the state of the boxed value, not the original unboxed value.

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.

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:

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

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 lost update problems (generally because an implicit boxing happened that was not taken into account in the code).

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.

Embedding Python

Note: 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.

The Python runtime assembly defines a number of public classes that provide a subset of the functionality provided by the Python C-API.

These classes include PyObject, PyList, PyDict, PyTuple, etc. You can review the nPython.exe source code in in “Console.csproj” project for example of embedding CPython in console .NET app. Please refer to this README GitHub page for new simplified embedding API:

README.md

At a very high level, to embed Python in your application you will need to:

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.

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.

Important Note for embedders: 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.

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.

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 PythonEngine.AcquireLock method. The only exception to this rule is the PythonEngine.Initialize method, which may be called at startup without having acquired the GIL.

When finished using Python APIs, managed code must call a corresponding PythonEngine.ReleaseLock to release the GIL and allow other threads to use Python.

The AcquireLock and ReleaseLock methods are thin wrappers over the unmanaged PyGILState_Ensure and PyGILState_Release functions from the Python API, and the documentation for those APIs applies to the managed versions.

License

Python for .NET is released under the open source MIT License. A copy of the license is included in the distribution, or you can find a copy of the license online.

Some distributions of this package include a copy of the C Python dlls and standard library, which are covered by the Python license.