Sunday, December 27, 2009

Adding Support for User-defined Classes

Believe it or not, classes were added late during Python’s first year of development at CWI, though well before the first public release. However, to understand how classes were added, it first helps to know a little bit about how Python is implemented.

Python is implemented in C as a classic stack-based byte code interpreter or “virtual machine” along with a collection of primitive types also implemented in C. The underlying architecture uses “objects” throughout, but since C has no direct support for objects, they are implemented using structures and function pointers. The Python virtual machine defines several dozen standard operations that every object type may or must implement (for example, “get attribute”, “add” and “call”). An object type is then represented by a statically allocated structure containing a series of function pointers, one for each standard operation. These function pointers are typically initialized with references to static functions. However, some operations are optional, and the object type may leave the function pointer NULL if it chooses not to implement that operation. In this case, the virtual machine either generates a run-time error or, in some cases, provides a default implementation of the operation. The type structure also contains various data fields, one of which is a reference to a list of additional methods that are unique to this type, represented as an array of structures containing a string (the method name) and a function pointer (its implementation) each. Python’s unique approach to introspection comes from its ability to make the type structure itself available at run-time as an object like all others.

An important aspect of this implementation is that it is completely C-centric. In fact, all of the standard operations and methods are implemented by C functions. Originally the byte code interpreter only supported calling pure Python functions and functions or methods implemented in C. I believe my colleague Siebren van der Zee was the first to suggest that Python should allow class definitions similar to those in C++ so that objects could also be implemented in Python.

To implement user-defined objects, I settled on the simplest possible design; a scheme where objects were represented by a new kind of built-in object that stored a class reference pointing to a "class object" shared by all instances of the same class, and a dictionary, dubbed the "instance dictionary", that contained the instance variables.

In this implementation, the instance dictionary would contain the instance variables of each individual object whereas the class object would contain stuff shared between all instances of the same class--in particular, methods. In implementing class objects, I again chose the simplest possible design; the set of methods of a class were stored in a dictionary whose keys are the method names. This, I dubbed the class dictionary. To support inheritance, class objects would additionally store a reference to the class objects corresponding to the base classes. At the time, I was fairly naïve about classes, but I knew about multiple inheritance, which had recently been added to C++. I decided that as long as I was going to support inheritance, I might as well support a simple-minded version of multiple inheritance. Thus, every class object could have one or more base classes.

In this implementation, the underlying mechanics of working with objects are actually very simple. Whenever changes are made to instance or class variables, those changes are simply reflected in the underlying dictionary object. For example, setting an instance variable on an instance updates its local instance dictionary. Likewise, when looking up the value of a instance variable of an object, one merely checks its instance dictionary for the existence of that variable. If the variable is not found there, things become a little more interesting. In that case, lookups are performed in the class dictionary and then in the class dictionaries of each of the base classes.

The process of looking up attributes in the class object and base classes is most commonly associated with locating methods. As previously mentioned, methods are stored in the dictionary of a class object which is shared by all instances of the same class. Thus, when a method is requested, you naturally won't find it in the instance dictionary of each individual object. Instead, you have to look it up in the class dictionary, and then ask each of the base classes in turn, stopping when a hit is found. Each of the base classes then implements the same algorithm recursively. This is commonly referred to as the depth-first, left-to-right rule, and has been the default method resolution order (MRO) used in most versions of Python. More modern releases have adapted a more sophisticated MRO, but that will be discussed in a later blog.

In implementing classes, one of my goals was to keep things simple. Thus, Python performs no advanced error checking or conformance checking when locating methods. For example, if a class overrides a method defined in a base class, no checks are performed to make sure that the redefined method has the same number of arguments or that it can be called in the same way as the original base-class method. The above method resolution algorithm merely returns the first method found and calls it with whatever arguments the user has supplied.

A number of other features also fall out of this design. For instance, even though the class dictionary was initially envisioned as a place to put methods, there was no inherent reason why other kinds of objects couldn't be placed there as well. Thus, if objects such as integers or strings are stored in the class dictionary, they become what are known as class variables---variables shared by all instances of a given class instead of being stored inside each instance.

Although the implementation of classes is simple, it also provides a surprisingly degree of flexibility. For instance, the implementation not only makes classes “first-class objects”, which are easily introspected at run time, it also makes it possible to modify a class dynamically. For example, methods can be added or modified by simply updating the class dictionary after a class object has already been created! (*) The dynamic nature of Python means that these changes have an immediate effect on all instances of that class or of any of its subclasses. Likewise, individual objects can be modified dynamically by adding, modifying, and deleting instance variables (a feature that I later learned made Python's implementation of objects more permissive than that found in Smalltalk which restricts the set of attributes to those specified at the time of object creation).

Development of the class Syntax

Having designed the run-time representations for user-defined classes and instances, my next task was to design the syntax for class definitions, and in particular for the method definitions contained therein. A major design constraint was that I didn’t want to add syntax for methods that differed from the syntax for functions. Refactoring the grammar and the byte code generator to handle such similar cases differently felt like a huge task. However, even if I was successful in keeping the grammar the same, I still had to figure out some way to deal with instance variables. Initially, I had hoped to emulate implicit instance variables as seen in C++. For example, in C++, classes are defined by code like this:
class A {
   int x;
   void spam(int y) {
        printf("%d %d\n", x, y);

In this class, an instance variable x has been declared. In methods, references to x implicitly refer to the corresponding instance variable. For example, in the method spam(), the variable x is not declared as either function parameter or as local variable However, since the class has declared an instance variable with that name, references to x simply refer to that variable. Although I had hoped to provide something similar in Python, it quickly became clear that such an approach would be impossible because there was no way to elegantly distinguish instance variables from local variables in a language without variable declarations.

In theory, getting the value of instance variables would be easy enough. Python already had a search order for unqualified variable names: locals, globals, and built-ins. Each of these were represented as a dictionary mapping variable names to values. Thus, for each variable reference, a series of dictionaries would be searched until a hit was found. For example, when executing a function with a local variable p, and a global variable q, a statement like “print p, q” would look up p in the first dictionary in the search order, the dictionary containing local variables, and find a match. Next it would look up q in the first dictionary, find no match, then look it up in the second dictionary, the global variables, and find a match.

It would have been easy to add the current object’s instance dictionary in front of this search list when executing a method. Then, in a method of an object with an instance variable x and local variable y, a statement like “print x, y” would find x in the instance dictionary (the first dictionary on the search path) and y in the local variable dictionary (the second dictionary).

The problem with this strategy is that it falls apart for setting instance variables. Python’s assignment doesn’t search for the variable name in the dictionaries, but simply adds or replaces the variable in the first dictionary in the search order, normally the local variables. This has the effect that variables are always created in the local scope by default (although it should be noted that there is a “global declaration” to override this on a per-function, per-variable basis.)

Without changing this simple-minded approach to assignment, making the instance dictionary the first item in the search order would make it impossible to assign to local variables in a method. For example, if you had a method like this
def spam(y):
    x = 1       
    y = 2       

The assignments to x and y would overwrite the instance variable x and create a new instance variable y that shadowed the local variable y. Swapping instance variables and local variables in the search order would merely reverse the problem, making it impossible to assign to instance variables.

Changing the semantics of assignment to assign to an instance variable if one already existed and to a local otherwise wouldn’t work either, since this would create a bootstrap problem: how does an instance variable get created initially? A possible solution might have been to require explicit declaration of instance variables as was the case for global variables, but I really didn’t want to add a feature like that given that that I had gotten this far without any variable declarations at all. Plus, the extra specification required for indicating a global variable was more of a special case that was used sparingly in most code. Requiring a special specification for instance variables, on the other hand, would have to be used almost everywhere in a class. Another possible solution would have been to make instance variables lexically distinct. For example, having instance variables start with a special character such as @ (an approach taken by Ruby) or by having some kind of special naming convention involving prefixes or capitalization. Neither of these appealed to me (and they still don't).

Instead, I decided to give up on the idea of implicit references to instance variables. Languages like C++ let you write this->foo to explicitly reference the instance variable foo (in case there’s a separate local variable foo). Thus, I decided to make such explicit references the only way to reference instance variables. In addition, I decided that rather than making the current object ("this") a special keyword, I would simply make "this" (or its equivalent) the first named argument to a method. Instance variables would just always be referenced as attributes of that argument.

With explicit references, there is no need to have a special syntax for method definitions nor do you have to worry about complicated semantics concerning variable lookup. Instead, one simply defines a function whose first argument corresponds to the instance, which by convention is named "self." For example:
def spam(self,y):
    print self.x, y

This approach resembles something I had seen in Modula-3, which had already provided me with the syntax for import and exception handling. Modula-3 doesn’t have classes, but it lets you create record types containing fully typed function pointer members that are initialized by default to functions defined nearby, and adds syntactic sugar so that if x is such a record variable, and m is a function pointer member of that record, initialized to function f, then calling x.m(args) is equivalent to calling f(x, args). This matches the typical implementation of objects and methods, and makes it possible to equate instance variables with attributes of the first argument.

The remaining details of Python’s class syntax follow from this design or from the constraints imposed by the rest of the implementation. Keeping with my desire for simplicity, I envisioned a class statement as a series of method definitions, which are syntactically identical to function definitions even though by convention, they are required to have a first argument named "self". In addition, rather than devising a new syntax for special kinds of class methods (such as initializers and destructors), I decided that these features could be handled by simply requiring the user to implement methods with special names such as __init__, __del__, and so forth. This naming convention was taken from C where identifiers starting with underscores are reserved by the compiler and often have special meaning (e.g., macros such as __FILE__ in the C preprocessor).

Thus, I envisioned that a class would be defined by code that looked like this:
class A:
     def __init__(self,x):
         self.x = x
     def spam(self,y):
        print self.x, y

Again, I wanted to reuse as much of my earlier code as possible. Normally, a function definition is an executable statement that simply sets a variable in the current namespace referencing the function object (the variable name is the function name). Thus, rather than coming up with an entirely different approach for handling classes, it made sense to me to simply interpret the class body as a series of statements that were executed in a new namespace. The dictionary of this namespace would then be captured and used to initialize the class dictionary and create a class object. Underneath the covers, the specific approach taken is to turn the class body into an anonymous function that executes all of the statements in the class body and then returns the resulting dictionary of local variables. This dictionary is then passed to a helper function that creates a class object. Finally, this class object is then stored in a variable in the surrounding scope, whose name is the class name. Users are often surprised to learn that any sequence of valid Python statements can appear in a class body. This capability was really just a straightforward extension of my desire to keep the syntax simple as well as not artificially limiting what might possibly be useful.

A final detail is the class instantiation (instance creation) syntax. Many languages, like C++ and Java, use a special operator, “new”, to create new class instances. In C++ this may be defensible since class names have a rather special status in the parser, but in Python this is not the case. I quickly realized that, since Python’s parser doesn’t care what kind of object you call, making the class object itself callable was the right, “minimal” solution, requiring no new syntax. I may have been ahead of my time here---today, factory functions are often the preferred pattern for instance creation, and what I had done was simply to turn each class into its own factory.

Special Methods

As briefly mentioned in the last section, one of my main goals was to keep the implementation of classes simple. In most object oriented languages, there are a variety of special operators and methods that only apply to classes. For example, in C++, there is a special syntax for defining constructors and destructors that is different than the normal syntax used to define ordinary function and methods.

I really didn't want to introduce additional syntax to handle special operations for objects. So instead, I handled this by simply mapping special operators to a predefined set of "special method" names such as __init__ and __del__. By defining methods with these names, users could supply code related to the construction and destruction of objects.

I also used this technique to allow user classes to redefine the behavior of Python's operators. As previously noted, Python is implemented in C and uses tables of function pointers to implement various capabilities of built-in objects (e.g., “get attribute”, “add” and “call”). To allow these capabilities to be defined in user-defined classes, I mapped the various function pointers to special method names such as __getattr__, __add__, and __call__. There is a direct correspondence between these names and the tables of function pointers one has to define when implementing new Python objects in C.

(*) Eventually, new-style classes made it necessary to control changes to the class __dict__; you can still dynamically modify a class, but you must use attribute assignment rather than using the class __dict__ directly.


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