This is Info file /home/pdm/tmp/Python-1.5.2p1/Doc/lib/python-lib.info, produced by Makeinfo version 1.68 from the input file lib.texi. July 6, 1999 1.5.2  File: python-lib.info, Node: Top, Next: Front Matter, Prev: (dir), Up: (dir) Python Library Reference ************************ * Menu: * Front Matter:: * Introduction:: * Built-in Objects:: * Python Services:: * String Services:: * Miscellaneous Services:: * Generic Operating System Services:: * Optional Operating System Services:: * Unix Specific Services:: * Python Debugger:: * Python Profiler:: * Internet Protocols and Support:: * Internet Data Handling:: * Restricted Execution:: * Multimedia Services:: * Cryptographic Services:: * SGI IRIX Specific Services:: * SunOS Specific Services:: * MS Windows Specific Services:: * Undocumented Modules:: * Module Index:: * Class-Exception-Object Index:: * Function-Method-Variable Index:: * Miscellaneous Index::  File: python-lib.info, Node: Front Matter, Next: Introduction, Prev: Top, Up: Top Front Matter ************ Copyright (C) 1991-1995 by Stichting Mathematisch Centrum, Amsterdam, The Netherlands. All Rights Reserved Permission to use, copy, modify, and distribute this software and its documentation for any purpose and without fee is hereby granted, provided that the above copyright notice appear in all copies and that both that copyright notice and this permission notice appear in supporting documentation, and that the names of Stichting Mathematisch Centrum or CWI or Corporation for National Research Initiatives or CNRI not be used in advertising or publicity pertaining to distribution of the software without specific, written prior permission. While CWI is the initial source for this software, a modified version is made available by the Corporation for National Research Initiatives (CNRI) at the Internet address `ftp://ftp.python.org'. STICHTING MATHEMATISCH CENTRUM AND CNRI DISCLAIM ALL WARRANTIES WITH REGARD TO THIS SOFTWARE, INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS, IN NO EVENT SHALL STICHTING MATHEMATISCH CENTRUM OR CNRI BE LIABLE FOR ANY SPECIAL, INDIRECT OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. Python is an extensible, interpreted, object-oriented programming language. It supports a wide range of applications, from simple text processing scripts to interactive WWW browsers. While the *Python Reference Manual* describes the exact syntax and semantics of the language, it does not describe the standard library that is distributed with the language, and which greatly enhances its immediate usability. This library contains built-in modules (written in C) that provide access to system functionality such as file I/O that would otherwise be inaccessible to Python programmers, as well as modules written in Python that provide standardized solutions for many problems that occur in everyday programming. Some of these modules are explicitly designed to encourage and enhance the portability of Python programs. This library reference manual documents Python's standard library, as well as many optional library modules (which may or may not be available, depending on whether the underlying platform supports them and on the configuration choices made at compile time). It also documents the standard types of the language and its built-in functions and exceptions, many of which are not or incompletely documented in the Reference Manual. This manual assumes basic knowledge about the Python language. For an informal introduction to Python, see the *Python Tutorial*; the *Python Reference Manual* remains the highest authority on syntactic and semantic questions. Finally, the manual entitled *Extending and Embedding the Python Interpreter* describes how to add new extensions to Python and how to embed it in other applications.  File: python-lib.info, Node: Introduction, Next: Built-in Objects, Prev: Front Matter, Up: Top Introduction ************ The "Python library" contains several different kinds of components. It contains data types that would normally be considered part of the "core" of a language, such as numbers and lists. For these types, the Python language core defines the form of literals and places some constraints on their semantics, but does not fully define the semantics. (On the other hand, the language core does define syntactic properties like the spelling and priorities of operators.) The library also contains built-in functions and exceptions -- objects that can be used by all Python code without the need of an `import' statement. Some of these are defined by the core language, but many are not essential for the core semantics and are only described here. The bulk of the library, however, consists of a collection of modules. There are many ways to dissect this collection. Some modules are written in C and built in to the Python interpreter; others are written in Python and imported in source form. Some modules provide interfaces that are highly specific to Python, like printing a stack trace; some provide interfaces that are specific to particular operating systems, like socket I/O; others provide interfaces that are specific to a particular application domain, like the World-Wide Web. Some modules are avaiable in all versions and ports of Python; others are only available when the underlying system supports or requires them; yet others are available only when a particular configuration option was chosen at the time when Python was compiled and installed. This manual is organized "from the inside out": it first describes the built-in data types, then the built-in functions and exceptions, and finally the modules, grouped in chapters of related modules. The ordering of the chapters as well as the ordering of the modules within each chapter is roughly from most relevant to least important. This means that if you start reading this manual from the start, and skip to the next chapter when you get bored, you will get a reasonable overview of the available modules and application areas that are supported by the Python library. Of course, you don't *have* to read it like a novel -- you can also browse the table of contents (in front of the manual), or look for a specific function, module or term in the index (in the back). And finally, if you enjoy learning about random subjects, you choose a random page number (see module `random') and read a section or two. Regardless of the order in which you read the sections of this manual, it helps to start with chapter *Note Built-in Objects::, "Built-in Types, Exceptions and Functions," as the remainder of the manual assumes familiarity with this material. Let the show begin!  File: python-lib.info, Node: Built-in Objects, Next: Python Services, Prev: Introduction, Up: Top Built-in Types, Exceptions and Functions **************************************** Names for built-in exceptions and functions are found in a separate symbol table. This table is searched last when the interpreter looks up the meaning of a name, so local and global user-defined names can override built-in names. Built-in types are described together here for easy reference.(1) The tables in this chapter document the priorities of operators by listing them in order of ascending priority (within a table) and grouping operators that have the same priority in the same box. Binary operators of the same priority group from left to right. (Unary operators group from right to left, but there you have no real choice.) See Chapter 5 of the *Python Reference Manual* for the complete picture on operator priorities. * Menu: * Built-in Types:: * Built-in Exceptions:: * Built-in Functions:: ---------- Footnotes ---------- (1) Most descriptions sorely lack explanations of the exceptions that may be raised -- this will be fixed in a future version of this manual.  File: python-lib.info, Node: Built-in Types, Next: Built-in Exceptions, Prev: Built-in Objects, Up: Built-in Objects Built-in Types ============== The following sections describe the standard types that are built into the interpreter. These are the numeric types, sequence types, and several others, including types themselves. There is no explicit Boolean type; use integers instead. Some operations are supported by several object types; in particular, all objects can be compared, tested for truth value, and converted to a string (with the ``...`' notation). The latter conversion is implicitly used when an object is written by the `print' statement. * Menu: * Truth Value Testing:: * Boolean Operations:: * Comparisons:: * Numeric Types:: * Sequence Types:: * Mapping Types:: * Other Built-in Types:: * Special Attributes::  File: python-lib.info, Node: Truth Value Testing, Next: Boolean Operations, Prev: Built-in Types, Up: Built-in Types Truth Value Testing ------------------- Any object can be tested for truth value, for use in an `if' or `while' condition or as operand of the Boolean operations below. The following values are considered false: * `None' * zero of any numeric type, e.g., `0', `0L', `0.0'. * any empty sequence, e.g., `''', `()', `[]'. * any empty mapping, e.g., `{}'. * instances of user-defined classes, if the class defines a `__nonzero__()' or `__len__()' method, when that method returns zero. All other values are considered true -- so objects of many types are always true. Operations and built-in functions that have a Boolean result always return `0' for false and `1' for true, unless otherwise stated. (Important exception: the Boolean operations `or' and `and' always return one of their operands.)  File: python-lib.info, Node: Boolean Operations, Next: Comparisons, Prev: Truth Value Testing, Up: Built-in Types Boolean Operations ------------------ These are the Boolean operations, ordered by ascending priority: Operation Result Notes ------ ----- ----- X or Y if X is false, then Y, (1) else X X and Y if X is false, then X, (1) else Y not X if X is false, then (2) `1', else `0' Notes: `(1)' These only evaluate their second argument if needed for their outcome. `(2)' `not' has a lower priority than non-Boolean operators, so e.g. `not a == b' is interpreted as `not(a == b)', and `a == not b' is a syntax error.  File: python-lib.info, Node: Comparisons, Next: Numeric Types, Prev: Boolean Operations, Up: Built-in Types Comparisons ----------- Comparison operations are supported by all objects. They all have the same priority (which is higher than that of the Boolean operations). Comparisons can be chained arbitrarily, e.g. `x < y <= z' is equivalent to `x < y and y <= z', except that `y' is evaluated only once (but in both cases `z' is not evaluated at all when `x < y' is found to be false). This table summarizes the comparison operations: Operation Meaning Notes ------ ----- ----- < strictly less than <= less than or equal > strictly greater than >= greater than or equal == equal <> not equal (1) != not equal (1) is object identity is not negated object identity Notes: `(1)' `<>' and `!=' are alternate spellings for the same operator. (I couldn't choose between ABC and C! :-) Objects of different types, except different numeric types, never compare equal; such objects are ordered consistently but arbitrarily (so that sorting a heterogeneous array yields a consistent result). Furthermore, some types (e.g., windows) support only a degenerate notion of comparison where any two objects of that type are unequal. Again, such objects are ordered arbitrarily but consistently. (Implementation note: objects of different types except numbers are ordered by their type names; objects of the same types that don't support proper comparison are ordered by their address.) Two more operations with the same syntactic priority, `in' and `not in', are supported only by sequence types (below).  File: python-lib.info, Node: Numeric Types, Next: Sequence Types, Prev: Comparisons, Up: Built-in Types Numeric Types ------------- There are four numeric types: "plain integers", "long integers", "floating point numbers", and "complex numbers". Plain integers (also just called "integers") are implemented using `long' in C, which gives them at least 32 bits of precision. Long integers have unlimited precision. Floating point numbers are implemented using `double' in C. All bets on their precision are off unless you happen to know the machine you are working with. Complex numbers have a real and imaginary part, which are both implemented using `double' in C. To extract these parts from a complex number Z, use `Z.real' and `Z.imag'. Numbers are created by numeric literals or as the result of built-in functions and operators. Unadorned integer literals (including hex and octal numbers) yield plain integers. Integer literals with an `L' or `l' suffix yield long integers (`L' is preferred because `1l' looks too much like eleven!). Numeric literals containing a decimal point or an exponent sign yield floating point numbers. Appending `j' or `J' to a numeric literal yields a complex number. Python fully supports mixed arithmetic: when a binary arithmetic operator has operands of different numeric types, the operand with the "smaller" type is converted to that of the other, where plain integer is smaller than long integer is smaller than floating point is smaller than complex. Comparisons between numbers of mixed type use the same rule.(1) The functions `int()', `long()', `float()', and `complex()' can be used to coerce numbers to a specific type. All numeric types support the following operations, sorted by ascending priority (operations in the same box have the same priority; all numeric operations have a higher priority than comparison operations): Operation Result Notes ------ ----- ----- X + Y sum of X and Y X - Y difference of X and Y X * Y product of X and Y X / Y quotient of X and Y (1) X %{} Y remainder of `X / Y' -X X negated +X X unchanged abs(X) absolute value or magnitude of X int(X) X converted to integer (2) long(X) X converted to long (2) integer float(X) X converted to floating point complex(RE,IM) a complex number with real part RE, imaginary part IM. IM defaults to zero. C.conjugate() conjugate of the complex number C divmod(X, Y) the pair `(X / Y, X %{} (3) Y)' pow(X, Y) X to the power Y X ** Y X to the power Y Notes: `(1)' For (plain or long) integer division, the result is an integer. The result is always rounded towards minus infinity: 1/2 is 0, (-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0. `(2)' Conversion from floating point to (long or plain) integer may round or truncate as in C; see functions `floor()' and `ceil()' in module `math' for well-defined conversions. `(3)' See the section on built-in functions for an exact definition. * Menu: * Bit-string Operations:: ---------- Footnotes ---------- (1) As a consequence, the list `[1, 2]' is considered equal to `[1.0, 2.0]', and similar for tuples.  File: python-lib.info, Node: Bit-string Operations, Prev: Numeric Types, Up: Numeric Types Bit-string Operations on Integer Types ...................................... Plain and long integer types support additional operations that make sense only for bit-strings. Negative numbers are treated as their 2's complement value (for long integers, this assumes a sufficiently large number of bits that no overflow occurs during the operation). The priorities of the binary bit-wise operations are all lower than the numeric operations and higher than the comparisons; the unary operation `~' has the same priority as the other unary numeric operations (`+' and `-'). This table lists the bit-string operations sorted in ascending priority (operations in the same box have the same priority): Operation Result Notes ------ ----- ----- X | Y bitwise "or" of X and Y X ^{} Y bitwise "exclusive or" of X and Y X &{} Y bitwise "and" of X and Y X << N X shifted left by N (1), (2) bits X >> N X shifted right by N (1), (3) bits ~X the bits of X inverted Notes: `(1)' Negative shift counts are illegal and cause a `ValueError' to be raised. `(2)' A left shift by N bits is equivalent to multiplication by `pow(2, N)' without overflow check. `(3)' A right shift by N bits is equivalent to division by `pow(2, N)' without overflow check.  File: python-lib.info, Node: Sequence Types, Next: Mapping Types, Prev: Numeric Types, Up: Built-in Types Sequence Types -------------- There are three sequence types: strings, lists and tuples. Strings literals are written in single or double quotes: `'xyzzy'', `"frobozz"'. See Chapter 2 of the *Python Reference Manual* for more about string literals. Lists are constructed with square brackets, separating items with commas: `[a, b, c]'. Tuples are constructed by the comma operator (not within square brackets), with or without enclosing parentheses, but an empty tuple must have the enclosing parentheses, e.g., `a, b, c' or `()'. A single item tuple must have a trailing comma, e.g., `(d,)'. Sequence types support the following operations. The `in' and `not in' operations have the same priorities as the comparison operations. The `+' and `*' operations have the same priority as the corresponding numeric operations.(1) This table lists the sequence operations sorted in ascending priority (operations in the same box have the same priority). In the table, S and T are sequences of the same type; N, I and J are integers: Operation Result Notes ------ ----- ----- X in S `1' if an item of S is equal to X, else `0' X not in S `0' if an item of S is equal to X, else `1' S + T the concatenation of S and T S * N, N * S N copies of S (3) concatenated S[I] I'th item of S, origin (1) 0 S[I:J] slice of S from I to J (1), (2) len(S) length of S min(S) smallest item of S max(S) largest item of S Notes: `(1)' If I or J is negative, the index is relative to the end of the string, i.e., `len(S) + I' or `len(S) + J' is substituted. But note that `-0' is still `0'. `(2)' The slice of S from I to J is defined as the sequence of items with index K such that `I <= K < J'. If I or J is greater than `len(S)', use `len(S)'. If I is omitted, use `0'. If J is omitted, use `len(S)'. If I is greater than or equal to J, the slice is empty. `(3)' Values of N less than `0' are treated as `0' (which yields an empty sequence of the same type as S). * Menu: * More String Operations:: * Mutable Sequence Types:: ---------- Footnotes ---------- (1) They must have since the parser can't tell the type of the operands.  File: python-lib.info, Node: More String Operations, Next: Mutable Sequence Types, Prev: Sequence Types, Up: Sequence Types More String Operations ...................... String objects have one unique built-in operation: the `%' operator (modulo) with a string left argument interprets this string as a C `sprintf()' format string to be applied to the right argument, and returns the string resulting from this formatting operation. The right argument should be a tuple with one item for each argument required by the format string; if the string requires a single argument, the right argument may also be a single non-tuple object.(1) The following format characters are understood: `%', `c', `s', `i', `d', `u', `o', `x', `X', `e', `E', `f', `g', `G'. Width and precision may be a `*' to specify that an integer argument specifies the actual width or precision. The flag characters `-', `+', blank, `#' and `0' are understood. The size specifiers `h', `l' or `L' may be present but are ignored. The `%s' conversion takes any Python object and converts it to a string using `str()' before formatting it. The ANSI features `%p' and `%n' are not supported. Since Python strings have an explicit length, `%s' conversions don't assume that `'\0'' is the end of the string. For safety reasons, floating point precisions are clipped to 50; `%f' conversions for numbers whose absolute value is over 1e25 are replaced by `%g' conversions.(2) All other errors raise exceptions. If the right argument is a dictionary (or any kind of mapping), then the formats in the string must have a parenthesized key into that dictionary inserted immediately after the `%' character, and each format formats the corresponding entry from the mapping. For example: >>> count = 2 >>> language = 'Python' >>> print '%(language)s has %(count)03d quote types.' % vars() Python has 002 quote types. In this case no `*' specifiers may occur in a format (since they require a sequential parameter list). Additional string operations are defined in standard module `string' and in built-in module `re'. ---------- Footnotes ---------- (1) A tuple object in this case should be a singleton. (2) These numbers are fairly arbitrary. They are intended to avoid printing endless strings of meaningless digits without hampering correct use and without having to know the exact precision of floating point values on a particular machine.  File: python-lib.info, Node: Mutable Sequence Types, Prev: More String Operations, Up: Sequence Types Mutable Sequence Types ...................... List objects support additional operations that allow in-place modification of the object. These operations would be supported by other mutable sequence types (when added to the language) as well. Strings and tuples are immutable sequence types and such objects cannot be modified once created. The following operations are defined on mutable sequence types (where X is an arbitrary object): Operation Result Notes ------ ----- ----- S[I] = X item I of S is replaced by X S[I:J] = T slice of S from I to J is replaced by T del S[I:J] same as `S[I:J] = []' S.append(X) same as `S[len(S):len(S)] = [X]' S.extend(X) same as (5) `S[len(S):len(S)] = X' S.count(X) return number of I's for which `S[I] == X' S.index(X) return smallest I such (1) that `S[I] == X' S.insert(I, X) same as `S[I:I] = [X]' if `I >= 0' S.pop([I]) same as `X = S[I]; del (4) S[I]; return X' S.remove(X) same as `del (1) S[S.index(X)]' S.reverse() reverses the items of S (3) in place S.sort([CMPFUNC]) sort the items of S in (2), (3) place {types} Notes: `(1)' Raises an exception when X is not found in S. `(2)' The `sort()' method takes an optional argument specifying a comparison function of two arguments (list items) which should return `-1', `0' or `1' depending on whether the first argument is considered smaller than, equal to, or larger than the second argument. Note that this slows the sorting process down considerably; e.g. to sort a list in reverse order it is much faster to use calls to the methods `sort()' and `reverse()' than to use the built-in function `sort()' with a comparison function that reverses the ordering of the elements. `(3)' The `sort()' and `reverse()' methods modify the list in place for economy of space when sorting or reversing a large list. They don't return the sorted or reversed list to remind you of this side effect. `(4)' The `pop()' method is experimental and not supported by other mutable sequence types than lists. The optional argument I defaults to `-1', so that by default the last item is removed and returned. `(5)' Raises an exception when X is not a list object. The `extend()' method is experimental and not supported by mutable types other than lists.  File: python-lib.info, Node: Mapping Types, Next: Other Built-in Types, Prev: Sequence Types, Up: Built-in Types Mapping Types ------------- A "mapping" object maps values of one type (the key type) to arbitrary objects. Mappings are mutable objects. There is currently only one standard mapping type, the "dictionary". A dictionary's keys are 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. 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 created by placing a comma-separated list of `KEY: VALUE' pairs within braces, for example: `{'jack': 4098, 'sjoerd': 4127}' or `{4098: 'jack', 4127: 'sjoerd'}'. The following operations are defined on mappings (where A and B are mappings, K is a key, and V and X are arbitrary objects): Operation Result Notes ------ ----- ----- len(A) the number of items in A A[K] the item of A with key (1) K A[K] = X set `A[K]' to X del A[K] remove `A[K]' from A (1) A.clear() remove all items from `a' A.copy() a (shallow) copy of `a' A.has_key(K) `1' if A has a key K, else `0' A.items() a copy of A's list of (2) (KEY, VALUE) pairs A.keys() a copy of A's list of (2) keys A.update(B) `for k, v in B.items(): (3) A[k] = v' A.values() a copy of A's list of (2) values A.get(K[, X]) `A[K]' if (4) `A.has_key(K)', else X Notes: `(1)' Raises a `KeyError' exception if K is not in the map. `(2)' Keys and values are listed in random order. `(3)' B must be of the same type as A. `(4)' Never raises an exception if K is not in the map, instead it returns F. F is optional; when F is not provided and K is not in the map, `None' is returned.  File: python-lib.info, Node: Other Built-in Types, Next: Special Attributes, Prev: Mapping Types, Up: Built-in Types Other Built-in Types -------------------- The interpreter supports several other kinds of objects. Most of these support only one or two operations. * Menu: * Modules:: * Classes and Instances:: * Functions:: * Methods:: * Code Objects:: * Type Objects:: * Null Object:: * Ellipsis Object:: * File Objectsfile:: * Internal Objects::  File: python-lib.info, Node: Modules, Next: Classes and Instances, Prev: Other Built-in Types, Up: Other Built-in Types Modules ....... The only special operation on a module is attribute access: `M.NAME', where M is a module and NAME accesses a name defined in M's symbol table. Module attributes can be assigned to. (Note that the `import' statement is not, strictly speaking, an operation on a module object; `import FOO' does not require a module object named FOO to exist, rather it requires an (external) *definition* for a module named FOO somewhere.) A special member of every module is `__dict__'. This is the dictionary containing the module's symbol table. Modifying this dictionary will actually change the module's symbol table, but direct assignment to the `__dict__' attribute is not possible (i.e., you can write `M.__dict__['a'] = 1', which defines `M.a' to be `1', but you can't write `M.__dict__ = {}'. Modules built into the interpreter are written like this: `'. If loaded from a file, they are written as `'.  File: python-lib.info, Node: Classes and Instances, Next: Functions, Prev: Modules, Up: Other Built-in Types Classes and Class Instances ........................... See Chapters 3 and 7 of the *Python Reference Manual* for these.  File: python-lib.info, Node: Functions, Next: Methods, Prev: Classes and Instances, Up: Other Built-in Types Functions ......... Function objects are created by function definitions. The only operation on a function object is to call it: `FUNC(ARGUMENT-LIST)'. There are really two flavors of function objects: built-in functions and user-defined functions. Both support the same operation (to call the function), but the implementation is different, hence the different object types. The implementation adds two special read-only attributes: `F.func_code' is a function's "code object" (see below) and `F.func_globals' is the dictionary used as the function's global name space (this is the same as `M.__dict__' where M is the module in which the function F was defined).  File: python-lib.info, Node: Methods, Next: Code Objects, Prev: Functions, Up: Other Built-in Types Methods ....... Methods are functions that are called using the attribute notation. There are two flavors: built-in methods (such as `append()' on lists) and class instance methods. Built-in methods are described with the types that support them. The implementation adds two special read-only attributes to class instance methods: `M.im_self' is the object on which the method operates, and `M.im_func' is the function implementing the method. Calling `M(ARG-1, ARG-2, ..., ARG-N)' is completely equivalent to calling `M.im_func(M.im_self, ARG-1, ARG-2, ..., ARG-N)'. See the *Python Reference Manual* for more information.  File: python-lib.info, Node: Code Objects, Next: Type Objects, Prev: Methods, Up: Other Built-in Types Code Objects ............ Code objects are used by the implementation to represent "pseudo-compiled" executable Python code such as a function body. They differ from function objects because they don't contain a reference to their global execution environment. Code objects are returned by the built-in `compile()' function and can be extracted from function objects through their `func_code' attribute. A code object can be executed or evaluated by passing it (instead of a source string) to the `exec' statement or the built-in `eval()' function. See the *Python Reference Manual* for more information.  File: python-lib.info, Node: Type Objects, Next: Null Object, Prev: Code Objects, Up: Other Built-in Types Type Objects ............ Type objects represent the various object types. An object's type is accessed by the built-in function `type()'. There are no special operations on types. The standard module `types' defines names for all standard built-in types. Types are written like this: `'.  File: python-lib.info, Node: Null Object, Next: Ellipsis Object, Prev: Type Objects, Up: Other Built-in Types The Null Object ............... This object is returned by functions that don't explicitly return a value. It supports no special operations. There is exactly one null object, named `None' (a built-in name). It is written as `None'.  File: python-lib.info, Node: Ellipsis Object, Next: File Objectsfile, Prev: Null Object, Up: Other Built-in Types The Ellipsis Object ................... This object is used by extended slice notation (see the *Python Reference Manual*). It supports no special operations. There is exactly one ellipsis object, named `Ellipsis' (a built-in name). It is written as `Ellipsis'.  File: python-lib.info, Node: File Objectsfile, Next: Internal Objects, Prev: Ellipsis Object, Up: Other Built-in Types File Objects........... File objects are implemented using C's `stdio' package and can be created with the built-in function `open()' described section *Note traceback::, "Built-in Functions." They are also returned by some other built-in functions and methods, e.g., `posix.popen()' and `posix.fdopen()' and the `makefile()' method of socket objects. When a file operation fails for an I/O-related reason, the exception `IOError' is raised. This includes situations where the operation is not defined for some reason, like `seek()' on a tty device or writing a file opened for reading. Files have the following methods: `close()' Close the file. A closed file cannot be read or written anymore. `flush()' Flush the internal buffer, like `stdio''s `fflush()'. `isatty()' Return `1' if the file is connected to a tty(-like) device, else `0'. `fileno()' Return the integer "file descriptor" that is used by the underlying implementation to request I/O operations from the operating system. This can be useful for other, lower level interfaces that use file descriptors, e.g. module `fcntl' or `os.read()' and friends. `read([size])' Read at most SIZE bytes from the file (less if the read hits `EOF' before obtaining SIZE bytes). If the SIZE argument is negative or omitted, read all data until `EOF' is reached. The bytes are returned as a string object. An empty string is returned when `EOF' is encountered immediately. (For certain files, like ttys, it makes sense to continue reading after an `EOF' is hit.) Note that this method may call the underlying C function `fread()' more than once in an effort to acquire as close to SIZE bytes as possible. `readline([size])' Read one entire line from the file. A trailing newline character is kept in the string(1) (but may be absent when a file ends with an incomplete line). If the SIZE argument is present and non-negative, it is a maximum byte count (including the trailing newline) and an incomplete line may be returned. An empty string is returned when `EOF' is hit immediately. Note: unlike `stdio''s `fgets()', the returned string contains null characters (`'\0'') if they occurred in the input. `readlines([sizehint])' Read until `EOF' using `readline()' and return a list containing the lines thus read. If the optional SIZEHINT argument is present, instead of reading up to `EOF', whole lines totalling approximately SIZEHINT bytes (possibly after rounding up to an internal buffer size) are read. `seek(offset[, whence])' Set the file's current position, like `stdio''s `fseek()'. The WHENCE argument is optional and defaults to `0' (absolute file positioning); other values are `1' (seek relative to the current position) and `2' (seek relative to the file's end). There is no return value. `tell()' Return the file's current position, like `stdio''s `ftell()'. `truncate([size])' Truncate the file's size. If the optional size argument present, the file is truncated to (at most) that size. The size defaults to the current position. Availability of this function depends on the operating system version (e.g., not all UNIX versions support this operation). `write(str)' Write a string to the file. There is no return value. Note: due to buffering, the string may not actually show up in the file until the `flush()' or `close()' method is called. `writelines(list)' Write a list of strings to the file. There is no return value. (The name is intended to match `readlines()'; `writelines()' does not add line separators.) File objects also offer the following attributes: `closed' Boolean indicating the current state of the file object. This is a read-only attribute; the `close()' method changes the value. `mode' The I/O mode for the file. If the file was created using the `open()' built-in function, this will be the value of the MODE parameter. This is a read-only attribute. `name' If the file object was created using `open()', the name of the file. Otherwise, some string that indicates the source of the file object, of the form `<...>'. This is a read-only attribute. `softspace' Boolean that indicates whether a space character needs to be printed before another value when using the `print' statement. Classes that are trying to simulate a file object should also have a writable `softspace' attribute, which should be initialized to zero. This will be automatic for classes implemented in Python; types implemented in C will have to provide a writable `softspace' attribute. ---------- Footnotes ---------- (1) The advantage of leaving the newline on is that an empty string can be returned to mean `EOF' without being ambiguous. Another advantage is that (in cases where it might matter, e.g. if you want to make an exact copy of a file while scanning its lines) you can tell whether the last line of a file ended in a newline or not (yes this happens!).  File: python-lib.info, Node: Internal Objects, Prev: File Objectsfile, Up: Other Built-in Types Internal Objects ................ See the *Python Reference Manual* for this information. It describes code objects, stack frame objects, traceback objects, and slice objects.  File: python-lib.info, Node: Special Attributes, Prev: Other Built-in Types, Up: Built-in Types Special Attributes ------------------ The implementation adds a few special read-only attributes to several object types, where they are relevant: `__dict__' A dictionary of some sort used to store an object's (writable) attributes. `__methods__' List of the methods of many built-in object types, e.g., `[].__methods__' yields `['append', 'count', 'index', 'insert', 'pop', 'remove', 'reverse', 'sort']'. `__members__' Similar to `__methods__', but lists data attributes. `__class__' The class to which a class instance belongs. `__bases__' The tuple of base classes of a class object.