# cpython-withatomic / Doc / ref3.tex

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All data in a Python program is represented by objects or by relations between objects. (In a sense, and in conformance to Von Neumann's model of a stored program computer'', code is also represented by objects.) \index{object} \index{data} Every object has an identity, a type and a value. An object's {\em identity} never changes once it has been created; you may think of it as the object's address in memory. An object's {\em type} is also unchangeable. It determines the operations that an object supports (e.g. does it have a length?'') and also defines the possible values for objects of that type. The {\em value} of some objects can change. Objects whose value can change are said to be {\em mutable}; objects whose value is unchangeable once they are created are called {\em immutable}. The type determines an object's (im)mutability. \index{identity of an object} \index{value of an object} \index{type of an object} \index{mutable object} \index{immutable object} Objects are never explicitly destroyed; however, when they become unreachable they may be garbage-collected. An implementation is allowed to delay garbage collection or omit it altogether --- it is a matter of implementation quality how garbage collection is implemented, as long as no objects are collected that are still reachable. (Implementation note: the current implementation uses a reference-counting scheme which collects most objects as soon as they become unreachable, but never collects garbage containing circular references.) \index{garbage collection} \index{reference counting} \index{unreachable object} Note that the use of the implementation's tracing or debugging facilities may keep objects alive that would normally be collectable. Some objects contain references to external'' resources such as open files or windows. It is understood that these resources are freed when the object is garbage-collected, but since garbage collection is not guaranteed to happen, such objects also provide an explicit way to release the external resource, usually a \verb@close@ method. Programs are strongly recommended to always explicitly close such objects. Some objects contain references to other objects; these are called {\em containers}. Examples of containers are tuples, lists and dictionaries. The references are part of a container's value. In most cases, when we talk about the value of a container, we imply the values, not the identities of the contained objects; however, when we talk about the (im)mutability of a container, only the identities of the immediately contained objects are implied. (So, if an immutable container contains a reference to a mutable object, its value changes if that mutable object is changed.) \index{container} Types affect almost all aspects of objects' lives. Even the meaning of object identity is affected in some sense: for immutable types, operations that compute new values may actually return a reference to any existing object with the same type and value, while for mutable objects this is not allowed. E.g. after \begin{verbatim} a = 1; b = 1; c = []; d = [] \end{verbatim} \verb@a@ and \verb@b@ may or may not refer to the same object with the value one, depending on the implementation, but \verb@c@ and \verb@d@ are guaranteed to refer to two different, unique, newly created empty lists. \section{The standard type hierarchy} \label{types} Below is a list of the types that are built into Python. Extension modules written in C can define additional types. Future versions of Python may add types to the type hierarchy (e.g. rational or complex numbers, efficiently stored arrays of integers, etc.). \index{type} \indexii{data}{type} \indexii{type}{hierarchy} \indexii{extension}{module} \index{C} Some of the type descriptions below contain a paragraph listing special attributes'. These are attributes that provide access to the implementation and are not intended for general use. Their definition may change in the future. There are also some generic' special attributes, not listed with the individual objects: \verb@__methods__@ is a list of the method names of a built-in object, if it has any; \verb@__members__@ is a list of the data attribute names of a built-in object, if it has any. \index{attribute} \indexii{special}{attribute} \indexiii{generic}{special}{attribute} \ttindex{__methods__} \ttindex{__members__} \begin{description} \item[None] This type has a single value. There is a single object with this value. This object is accessed through the built-in name \verb@None@. It is returned from functions that don't explicitly return an object. \ttindex{None} \obindex{None@{\tt None}} \item[Numbers] These are created by numeric literals and returned as results by arithmetic operators and arithmetic built-in functions. Numeric objects are immutable; once created their value never changes. Python numbers are of course strongly related to mathematical numbers, but subject to the limitations of numerical representation in computers. \obindex{number} \obindex{numeric} Python distinguishes between integers and floating point numbers: \begin{description} \item[Integers] These represent elements from the mathematical set of whole numbers. \obindex{integer} There are two types of integers: \begin{description} \item[Plain integers] These represent numbers in the range $-2^{31}$ through $2^{31}-1$. (The range may be larger on machines with a larger natural word size, but not smaller.) When the result of an operation falls outside this range, the exception \verb@OverflowError@ is raised. For the purpose of shift and mask operations, integers are assumed to have a binary, 2's complement notation using 32 or more bits, and hiding no bits from the user (i.e., all $2^{32}$ different bit patterns correspond to different values). \obindex{plain integer} \item[Long integers] These represent numbers in an unlimited range, subject to available (virtual) memory only. For the purpose of shift and mask operations, a binary representation is assumed, and negative numbers are represented in a variant of 2's complement which gives the illusion of an infinite string of sign bits extending to the left. \obindex{long integer} \end{description} % Integers The rules for integer representation are intended to give the most meaningful interpretation of shift and mask operations involving negative integers and the least surprises when switching between the plain and long integer domains. For any operation except left shift, if it yields a result in the plain integer domain without causing overflow, it will yield the same result in the long integer domain or when using mixed operands. \indexii{integer}{representation} \item[Floating point numbers] These represent machine-level double precision floating point numbers. You are at the mercy of the underlying machine architecture and C implementation for the accepted range and handling of overflow. \obindex{floating point} \indexii{floating point}{number} \index{C} \end{description} % Numbers \item[Sequences] These represent finite ordered sets indexed by natural numbers. The built-in function \verb@len()@ returns the number of elements of a sequence. When this number is $n$, the index set contains the numbers $0, 1, \ldots, n-1$. Element \verb@i@ of sequence \verb@a@ is selected by \verb@a[i]@. \obindex{seqence} \bifuncindex{len} \index{index operation} \index{item selection} \index{subscription} Sequences also support slicing: \verb@a[i:j]@ selects all elements with index $k$ such that $i <= k < j$. When used as an expression, a slice is a sequence of the same type --- this implies that the index set is renumbered so that it starts at 0 again. \index{slicing} Sequences are distinguished according to their mutability: \begin{description} % \item[Immutable sequences] An object of an immutable sequence type cannot change once it is created. (If the object contains references to other objects, these other objects may be mutable and may be changed; however the collection of objects directly referenced by an immutable object cannot change.) \obindex{immutable sequence} \obindex{immutable} The following types are immutable sequences: \begin{description} \item[Strings] The elements of a string are characters. There is no separate character type; a character is represented by a string of one element. Characters represent (at least) 8-bit bytes. The built-in functions \verb@chr()@ and \verb@ord()@ convert between characters and nonnegative integers representing the byte values. Bytes with the values 0-127 represent the corresponding ASCII values. The string data type is also used to represent arrays of bytes, e.g. to hold data read from a file. \obindex{string} \index{character} \index{byte} \index{ASCII} \bifuncindex{chr} \bifuncindex{ord} (On systems whose native character set is not ASCII, strings may use EBCDIC in their internal representation, provided the functions \verb@chr()@ and \verb@ord()@ implement a mapping between ASCII and EBCDIC, and string comparison preserves the ASCII order. Or perhaps someone can propose a better rule?) \index{ASCII} \index{EBCDIC} \index{character set} \indexii{string}{comparison} \bifuncindex{chr} \bifuncindex{ord} \item[Tuples] The elements of a tuple are arbitrary Python objects. Tuples of two or more elements are formed by comma-separated lists of expressions. A tuple of one element (a singleton') can be formed by affixing a comma to an expression (an expression by itself does not create a tuple, since parentheses must be usable for grouping of expressions). An empty tuple can be formed by enclosing nothing' in parentheses. \obindex{tuple} \indexii{singleton}{tuple} \indexii{empty}{tuple} \end{description} % Immutable sequences \item[Mutable sequences] Mutable sequences can be changed after they are created. The subscription and slicing notations can be used as the target of assignment and \verb@del@ (delete) statements. \obindex{mutable sequece} \obindex{mutable} \indexii{assignment}{statement} \index{delete} \stindex{del} \index{subscription} \index{slicing} There is currently a single mutable sequence type: \begin{description} \item[Lists] The elements of a list are arbitrary Python objects. Lists are formed by placing a comma-separated list of expressions in square brackets. (Note that there are no special cases needed to form lists of length 0 or 1.) \obindex{list} \end{description} % Mutable sequences \end{description} % Sequences \item[Mapping types] These represent finite sets of objects indexed by arbitrary index sets. The subscript notation \verb@a[k]@ selects the element indexed by \verb@k@ from the mapping \verb@a@; this can be used in expressions and as the target of assignments or \verb@del@ statements. The built-in function \verb@len()@ returns the number of elements in a mapping. \bifuncindex{len} \index{subscription} \obindex{mapping} There is currently a single mapping type: \begin{description} \item[Dictionaries] These represent finite sets of objects indexed by almost arbitrary values. The only types of values not acceptable as keys are values containing lists or dictionaries or other mutable types that are compared by value rather than by object identity --- the reason being that the implementation requires that a key's hash value be constant. Numeric types used for keys obey the normal rules for numeric comparison: if two numbers compare equal (e.g. 1 and 1.0) then they can be used interchangeably to index the same dictionary entry. Dictionaries are mutable; they are created by the \verb@{...}@ notation (see section \ref{dict}). \obindex{dictionary} \obindex{mutable} \end{description} % Mapping types \item[Callable types] These are the types to which the function call (invocation) operation, written as \verb@function(argument, argument, ...)@, can be applied: \indexii{function}{call} \index{invocation} \indexii{function}{argument} \obindex{callable} \begin{description} \item[User-defined functions] A user-defined function object is created by a function definition (see section \ref{function}). It should be called with an argument list containing the same number of items as the function's formal parameter list. \indexii{user-defined}{function} \obindex{function} \obindex{user-defined function} Special read-only attributes: \verb@func_code@ is the code object representing the compiled function body, and \verb@func_globals@ is (a reference to) the dictionary that holds the function's global variables --- it implements the global name space of the module in which the function was defined. \ttindex{func_code} \ttindex{func_globals} \indexii{global}{name space} \item[User-defined methods] A user-defined method (a.k.a. {\em object closure}) is a pair of a class instance object and a user-defined function. It should be called with an argument list containing one item less than the number of items in the function's formal parameter list. When called, the class instance becomes the first argument, and the call arguments are shifted one to the right. \obindex{method} \obindex{user-defined method} \indexii{user-defined}{method} \index{object closure} Special read-only attributes: \verb@im_self@ is the class instance object, \verb@im_func@ is the function object. \ttindex{im_func} \ttindex{im_self} \item[Built-in functions] A built-in function object is a wrapper around a C function. Examples of built-in functions are \verb@len@ and \verb@math.sin@. There are no special attributes. The number and type of the arguments are determined by the C function. \obindex{built-in function} \obindex{function} \index{C} \item[Built-in methods] This is really a different disguise of a built-in function, this time containing an object passed to the C function as an implicit extra argument. An example of a built-in method is \verb@list.append@ if \verb@list@ is a list object. \obindex{built-in method} \obindex{method} \indexii{built-in}{method} \item[Classes] Class objects are described below. When a class object is called as a function, a new class instance (also described below) is created and returned. This implies a call to the class's \verb@__init__@ method if it has one. Any arguments are passed on to the \verb@__init__@ method -- if there is \verb@__init__@ method, the class must be called without arguments. \ttindex{__init__} \obindex{class} \obindex{class instance} \obindex{instance} \indexii{class object}{call} \end{description} \item[Modules] Modules are imported by the \verb@import@ statement (see section \ref{import}). A module object is a container for a module's name space, which is a dictionary (the same dictionary as referenced by the \verb@func_globals@ attribute of functions defined in the module). Module attribute references are translated to lookups in this dictionary. A module object does not contain the code object used to initialize the module (since it isn't needed once the initialization is done). \stindex{import} \obindex{module} Attribute assignment update the module's name space dictionary. Special read-only attributes: \verb@__dict__@ yields the module's name space as a dictionary object; \verb@__name__@ yields the module's name as a string object. \ttindex{__dict__} \ttindex{__name__} \indexii{module}{name space} \item[Classes] Class objects are created by class definitions (see section \ref{class}). A class is a container for a dictionary containing the class's name space. Class attribute references are translated to lookups in this dictionary. When an attribute name is not found there, the attribute search continues in the base classes. The search is depth-first, left-to-right in the order of their occurrence in the base class list. \obindex{class} \obindex{class instance} \obindex{instance} \indexii{class object}{call} \index{container} \obindex{dictionary} \indexii{class}{attribute} Class attribute assignments update the class's dictionary, never the dictionary of a base class. \indexiii{class}{attribute}{assignment} A class can be called as a function to yield a class instance (see above). \indexii{class object}{call} Special read-only attributes: \verb@__dict__@ yields the dictionary containing the class's name space; \verb@__bases__@ yields a tuple (possibly empty or a singleton) containing the base classes, in the order of their occurrence in the base class list. \ttindex{__dict__} \ttindex{__bases__} \item[Class instances] A class instance is created by calling a class object as a function. A class instance has a dictionary in which attribute references are searched. When an attribute is not found there, and the instance's class has an attribute by that name, and that class attribute is a user-defined function (and in no other cases), the instance attribute reference yields a user-defined method object (see above) constructed from the instance and the function. \obindex{class instance} \obindex{instance} \indexii{class}{instance} \indexii{class instance}{attribute} Attribute assignments update the instance's dictionary. \indexiii{class instance}{attribute}{assignment} Class instances can pretend to be numbers, sequences, or mappings if they have methods with certain special names. These are described in section \ref{specialnames}. \obindex{number} \obindex{sequence} \obindex{mapping} Special read-only attributes: \verb@__dict__@ yields the attribute dictionary; \verb@__class__@ yields the instance's class. \ttindex{__dict__} \ttindex{__class__} \item[Files] A file object represents an open file. (It is a wrapper around a C {\tt stdio} file pointer.) File objects are created by the \verb@open()@ built-in function, and also by \verb@posix.popen()@ and the \verb@makefile@ method of socket objects. \verb@sys.stdin@, \verb@sys.stdout@ and \verb@sys.stderr@ are file objects corresponding the the interpreter's standard input, output and error streams. See the Python Library Reference for methods of file objects and other details. \obindex{file} \index{C} \index{stdio} \bifuncindex{open} \bifuncindex{popen} \bifuncindex{makefile} \ttindex{stdin} \ttindex{stdout} \ttindex{stderr} \ttindex{sys.stdin} \ttindex{sys.stdout} \ttindex{sys.stderr} \item[Internal types] A few types used internally by the interpreter are exposed to the user. Their definition may change with future versions of the interpreter, but they are mentioned here for completeness. \index{internal type} \begin{description} \item[Code objects] Code objects represent executable code. The difference between a code object and a function object is that the function object contains an explicit reference to the function's context (the module in which it was defined) which a code object contains no context. There is no way to execute a bare code object. \obindex{code} Special read-only attributes: \verb@co_code@ is a string representing the sequence of instructions; \verb@co_consts@ is a list of literals used by the code; \verb@co_names@ is a list of names (strings) used by the code; \verb@co_filename@ is the filename from which the code was compiled. (To find out the line numbers, you would have to decode the instructions; the standard library module \verb@dis@ contains an example of how to do this.) \ttindex{co_code} \ttindex{co_consts} \ttindex{co_names} \ttindex{co_filename} \item[Frame objects] Frame objects represent execution frames. They may occur in traceback objects (see below). \obindex{frame} Special read-only attributes: \verb@f_back@ is to the previous stack frame (towards the caller), or \verb@None@ if this is the bottom stack frame; \verb@f_code@ is the code object being executed in this frame; \verb@f_globals@ is the dictionary used to look up global variables; \verb@f_locals@ is used for local variables; \verb@f_lineno@ gives the line number and \verb@f_lasti@ gives the precise instruction (this is an index into the instruction string of the code object). \ttindex{f_back} \ttindex{f_code} \ttindex{f_globals} \ttindex{f_locals} \ttindex{f_lineno} \ttindex{f_lasti} \item[Traceback objects] \label{traceback} Traceback objects represent a stack trace of an exception. A traceback object is created when an exception occurs. When the search for an exception handler unwinds the execution stack, at each unwound level a traceback object is inserted in front of the current traceback. When an exception handler is entered (see also section \ref{try}), the stack trace is made available to the program as \verb@sys.exc_traceback@. When the program contains no suitable handler, the stack trace is written (nicely formatted) to the standard error stream; if the interpreter is interactive, it is also made available to the user as \verb@sys.last_traceback@. \obindex{traceback} \indexii{stack}{trace} \indexii{exception}{handler} \indexii{execution}{stack} \ttindex{exc_traceback} \ttindex{last_traceback} \ttindex{sys.exc_traceback} \ttindex{sys.last_traceback} Special read-only attributes: \verb@tb_next@ is the next level in the stack trace (towards the frame where the exception occurred), or \verb@None@ if there is no next level; \verb@tb_frame@ points to the execution frame of the current level; \verb@tb_lineno@ gives the line number where the exception occurred; \verb@tb_lasti@ indicates the precise instruction. The line number and last instruction in the traceback may differ from the line number of its frame object if the exception occurred in a \verb@try@ statement with no matching \verb@except@ clause or with a \verb@finally@ clause. \ttindex{tb_next} \ttindex{tb_frame} \ttindex{tb_lineno} \ttindex{tb_lasti} \stindex{try} \end{description} % Internal types \end{description} % Types \section{Special method names} \label{specialnames} A class can implement certain operations that are invoked by special syntax (such as subscription or arithmetic operations) by defining methods with special names. For instance, if a class defines a method named \verb@__getitem__@, and \verb@x@ is an instance of this class, then \verb@x[i]@ is equivalent to \verb@x.__getitem__(i)@. (The reverse is not true --- if \verb@x@ is a list object, \verb@x.__getitem__(i)@ is not equivalent to \verb@x[i]@.) Except for \verb@__repr__@, \verb@__str__@ and \verb@__cmp__@, attempts to execute an operation raise an exception when no appropriate method is defined. For \verb@__repr__@, the default is to return a string describing the object's class and address. For \verb@__cmp__@, the default is to compare instances based on their address. For \verb@__str__@, the default is to use \verb@__repr__@. \subsection{Special methods for any type} \begin{description} \item[\tt __init__(self, args...)] Called when the instance is created. The arguments are those passed to the class constructor expression. If a base class has an \code{__init__} method the derived class's \code{__init__} method must explicitly call it to ensure proper initialization of the base class part of the instance. \item[\tt __del__(self)] Called when the instance is about to be destroyed. If a base class has an \code{__del__} method the derived class's \code{__del__} method must explicitly call it to ensure proper deletion of the base class part of the instance. Note that it is possible for the \code{__del__} method to postpone destruction of the instance by creating a new reference to it. It may then be called at a later time when this new reference is deleted. It is not guaranteed that \code{__del__} methods are called for objects that still exist when the interpreter exits. Note that \code{del x} doesn't directly call \code{x.__del__} -- the former decrements the reference count for \code{x} by one, but \code{x,__del__} is only called when its reference count reaches zero. \item[\tt __repr__(self)] Called by the \verb@repr()@ built-in function and by conversions (reverse quotes) to compute the string representation of an object. \item[\tt __str__(self)] Called by the \verb@str()@ built-in function and by the \verb@print@ statement compute the string representation of an object. \item[\tt __cmp__(self, other)] Called by all comparison operations. Should return -1 if \verb@self < other@, 0 if \verb@self == other@, +1 if \verb@self > other@. If no \code{__cmp__} operation is defined, class instances are compared by object identity (address''). (Implementation note: due to limitations in the interpreter, exceptions raised by comparisons are ignored, and the objects will be considered equal in this case.) \item[\tt __hash__(self)] Called for the key object for dictionary operations, and by the built-in function \code{hash()}. Should return a 32-bit integer usable as a hash value for dictionary operations. The only required property is that objects which compare equal have the same hash value; it is advised to somehow mix together (e.g. using exclusing or) the hash values for the components of the object that also play a part in comparison of objects. If a class does not define a \code{__cmp__} method it should not define a \code{__hash__} operation either; if it defines \code{__cmp__} but not \code{__hash__} its instances will not be usable as dictionary keys. If a class defines mutable objects and implements a \code{__cmp__} method it should not implement \code{__hash__}, since the dictionary implementation assumes that a key's hash value is a constant. \obindex{dictionary} \item[\tt __call__(self, *args)] Called when the instance is called'' as a function. \end{description} \subsection{Special methods for attribute access} The following methods can be used to change the meaning of attribute access for class instances. \begin{description} \item[\tt __getattr__(self, name)] Called when an attribute lookup has not found the attribute in the usual places (i.e. it is not an instance attribute nor is it found in the class tree for \code{self}). \code{name} is the attribute name. Note that if the attribute is found through the normal mechanism, \code{__getattr__} is not called. (This is an asymmetry between \code{__getattr__} and \code{__setattr__}.) This is done both for efficiency reasons and because otherwise \code{__getattr__} would have no way to access other attributes of the instance. Note that at least for instance variables, \code{__getattr__} can fake total control by simply not inserting any values in the instance attribute dictionary. \item[\tt __setattr__(self, name, value)] Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value as an instance attribute). \code{name} is the attribute name, \code{value} is the value to be assigned to it. If \code{__setattr__} wants to assign to an instance attribute, it should not simply execute \code{self.\var{name} = value} -- this would cause a recursive call. Instead, it should insert the value in the dictionary of instance attributes, e.g. \code{self.__dict__[name] = value}. \item[\tt __delattr__(self, name)] Like \code{__setattr__} but for attribute deletion instead of assignment. \end{description} \subsection{Special methods for sequence and mapping types} \begin{description} \item[\tt __len__(self)] Called to implement the built-in function \verb@len()@. Should return the length of the object, an integer \verb@>=@ 0. Also, an object whose \verb@__len__()@ method returns 0 is considered to be false in a Boolean context. \item[\tt __getitem__(self, key)] Called to implement evaluation of \verb@self[key]@. Note that the special interpretation of negative keys (if the class wishes to emulate a sequence type) is up to the \verb@__getitem__@ method. \item[\tt __setitem__(self, key, value)] Called to implement assignment to \verb@self[key]@. Same note as for \verb@__getitem__@. \item[\tt __delitem__(self, key)] Called to implement deletion of \verb@self[key]@. Same note as for \verb@__getitem__@. \end{description} \subsection{Special methods for sequence types} \begin{description} \item[\tt __getslice__(self, i, j)] Called to implement evaluation of \verb@self[i:j]@. Note that missing \verb@i@ or \verb@j@ are replaced by 0 or \verb@len(self)@, respectively, and \verb@len(self)@ has been added (once) to originally negative \verb@i@ or \verb@j@ by the time this function is called (unlike for \verb@__getitem__@). \item[\tt __setslice__(self, i, j, sequence)] Called to implement assignment to \verb@self[i:j]@. Same notes as for \verb@__getslice__@. \item[\tt __delslice__(self, i, j)] Called to implement deletion of \verb@self[i:j]@. Same notes as for \verb@__getslice__@. \end{description} \subsection{Special methods for numeric types} \begin{description} \item[\tt __add__(self, other)]\itemjoin \item[\tt __sub__(self, other)]\itemjoin \item[\tt __mul__(self, other)]\itemjoin \item[\tt __div__(self, other)]\itemjoin \item[\tt __mod__(self, other)]\itemjoin \item[\tt __divmod__(self, other)]\itemjoin \item[\tt __pow__(self, other)]\itemjoin \item[\tt __lshift__(self, other)]\itemjoin \item[\tt __rshift__(self, other)]\itemjoin \item[\tt __and__(self, other)]\itemjoin \item[\tt __xor__(self, other)]\itemjoin \item[\tt __or__(self, other)]\itembreak Called to implement the binary arithmetic operations (\verb@+@, \verb@-@, \verb@*@, \verb@/@, \verb@%@, \verb@divmod()@, \verb@pow()@, \verb@<<@, \verb@>>@, \verb@&@, \verb@^@, \verb@|@). \item[\tt __neg__(self)]\itemjoin \item[\tt __pos__(self)]\itemjoin \item[\tt __abs__(self)]\itemjoin \item[\tt __invert__(self)]\itembreak Called to implement the unary arithmetic operations (\verb@-@, \verb@+@, \verb@abs()@ and \verb@~@). \item[\tt __nonzero__(self)] Called to implement boolean testing; should return 0 or 1. An alternative name for this method is \verb@__len__@. \item[\tt __coerce__(self, other)] Called to implement mixed-mode'' numeric arithmetic. Should either return a tuple containing self and other converted to a common numeric type, or None if no way of conversion is known. When the common type would be the type of other, it is sufficient to return None, since the interpreter will also ask the other object to attempt a coercion (but sometimes, if the implementation of the other type cannot be changed, it is useful to do the conversion to the other type here). Note that this method is not called to coerce the arguments to \verb@+@ and \verb@*@, because these are also used to implement sequence concatenation and repetition, respectively. Also note that, for the same reason, in \verb@n*x@, where \verb@n@ is a built-in number and \verb@x@ is an instance, a call to \verb@x.__mul__(n)@ is made.% \footnote{The interpreter should really distinguish between user-defined classes implementing sequences, mappings or numbers, but currently it doesn't --- hence this strange exception.} \item[\tt __int__(self)]\itemjoin \item[\tt __long__(self)]\itemjoin \item[\tt __float__(self)]\itembreak Called to implement the built-in functions \verb@int()@, \verb@long()@ and \verb@float()@. Should return a value of the appropriate type. \item[\tt __oct__(self)]\itemjoin \item[\tt __hex__(self)]\itembreak Called to implement the built-in functions \verb@oct()@ and \verb@hex()@. Should return a string value. \end{description}