This is /home/pdm/install/Python-2.1/Doc/lib/python-lib.info, produced by makeinfo version 4.0 from lib.texi. April 15, 2001 2.1  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 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 ). 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 in section *Note Built-in Functions::, "Built-in Functions." They are also returned by some other built-in functions and methods, e.g., `os.popen()' and `os.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. Any operation which requires that the file be open will raise a `ValueError' after the file has been closed. Calling `close()' more than once is allowed. `flush()' Flush the internal buffer, like `stdio''s `fflush()'. This may be a no-op on some file-like objects. `isatty()' Return true if the file is connected to a tty(-like) device, else false. *Note:* If a file-like object is not associated with a real file, this method should _not_ be implemented. `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. *Note:* File-like objects which do not have a real file descriptor should _not_ provide this method! `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. Objects implementing a file-like interface may choose to ignore SIZEHINT if it cannot be implemented, or cannot be implemented efficiently. `xreadlines()' Equivalent to `xreadlines.xreadlines(file)'. `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. Note that if the file is opened for appending (mode `'a'' or `'a+''), any `seek()' operations will be undone at the next write. If the file is only opened for writing in append mode (mode `'a''), this method is essentially a no-op, but it remains useful for files opened in append mode with reading enabled (mode `'a+''). `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 (for example, 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.) `xreadlines()' Equivalent to `xreadlines.xreadlines(FILE)'.(See the `xreadlines' module for more information.) File objects also offer a number of other interesting attributes. These are not required for file-like objects, but should be implemented if they make sense for the particular object. `closed' Boolean indicating the current state of the file object. This is a read-only attribute; the `close()' method changes the value. It may not be available on all file-like objects. `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 and may not be present on all file-like objects. `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 and may not be present on all file-like objects. `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 most classes implemented in Python (care may be needed for objects that override attribute access); types implemented in C will have to provide a writable `softspace' attribute. *Note:* This attribute is not used to control the `print' statement, but to allow the implementation of `print' to keep track of its internal state. ---------- 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 for this information. It describes 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 or other mapping object 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']'. This usually does not need to be explicitly provided by the object. `__members__' Similar to `__methods__', but lists data attributes. This usually does not need to be explicitly provided by the object. `__class__' The class to which a class instance belongs. `__bases__' The tuple of base classes of a class object.  File: python-lib.info, Node: Built-in Exceptions, Next: Built-in Functions, Prev: Built-in Types, Up: Built-in Objects Built-in Exceptions =================== Standard exceptions classes. Exceptions can be class objects or string objects. Though most exceptions have been string objects in past versions of Python, in Python 1.5 and newer versions, all standard exceptions have been converted to class objects, and users are encouraged to do the same. The exceptions are defined in the module `exceptions'. This module never needs to be imported explicitly: the exceptions are provided in the built-in namespace. Two distinct string objects with the same value are considered different exceptions. This is done to force programmers to use exception names rather than their string value when specifying exception handlers. The string value of all built-in exceptions is their name, but this is not a requirement for user-defined exceptions or exceptions defined by library modules. For class exceptions, in a `try' statement with an `except' clause that mentions a particular class, that clause also handles any exception classes derived from that class (but not exception classes from which _it_ is derived). Two exception classes that are not related via subclassing are never equivalent, even if they have the same name. The built-in exceptions listed below can be generated by the interpreter or built-in functions. Except where mentioned, they have an "associated value" indicating the detailed cause of the error. This may be a string or a tuple containing several items of information (e.g., an error code and a string explaining the code). The associated value is the second argument to the `raise' statement. For string exceptions, the associated value itself will be stored in the variable named as the second argument of the `except' clause (if any). For class exceptions, that variable receives the exception instance. If the exception class is derived from the standard root class `Exception', the associated value is present as the exception instance's `args' attribute, and possibly on other attributes as well. User code can raise built-in exceptions. This can be used to test an exception handler or to report an error condition "just like" the situation in which the interpreter raises the same exception; but beware that there is nothing to prevent user code from raising an inappropriate error. The following exceptions are only used as base classes for other exceptions. `Exception' The root class for exceptions. All built-in exceptions are derived from this class. All user-defined exceptions should also be derived from this class, but this is not (yet) enforced. The `str()' function, when applied to an instance of this class (or most derived classes) returns the string value of the argument or arguments, or an empty string if no arguments were given to the constructor. When used as a sequence, this accesses the arguments given to the constructor (handy for backward compatibility with old code). The arguments are also available on the instance's `args' attribute, as a tuple. `StandardError' The base class for all built-in exceptions except `SystemExit'. `StandardError' itself is derived from the root class `Exception'. `ArithmeticError' The base class for those built-in exceptions that are raised for various arithmetic errors: `OverflowError', `ZeroDivisionError', `FloatingPointError'. `LookupError' The base class for the exceptions that are raised when a key or index used on a mapping or sequence is invalid: `IndexError', `KeyError'. This can be raised directly by `sys.setdefaultencoding()'. `EnvironmentError' The base class for exceptions that can occur outside the Python system: `IOError', `OSError'. When exceptions of this type are created with a 2-tuple, the first item is available on the instance's `errno' attribute (it is assumed to be an error number), and the second item is available on the `strerror' attribute (it is usually the associated error message). The tuple itself is also available on the `args' attribute. _Added in Python version 1.5.2_ When an `EnvironmentError' exception is instantiated with a 3-tuple, the first two items are available as above, while the third item is available on the `filename' attribute. However, for backwards compatibility, the `args' attribute contains only a 2-tuple of the first two constructor arguments. The `filename' attribute is `None' when this exception is created with other than 3 arguments. The `errno' and `strerror' attributes are also `None' when the instance was created with other than 2 or 3 arguments. In this last case, `args' contains the verbatim constructor arguments as a tuple. The following exceptions are the exceptions that are actually raised. `AssertionError' Raised when an `assert' statement fails. `AttributeError' Raised when an attribute reference or assignment fails. (When an object does not support attribute references or attribute assignments at all, `TypeError' is raised.) `EOFError' Raised when one of the built-in functions (`input()' or `raw_input()') hits an end-of-file condition (`EOF') without reading any data. (N.B.: the `read()' and `readline()' methods of file objects return an empty string when they hit `EOF'.) `FloatingPointError' Raised when a floating point operation fails. This exception is always defined, but can only be raised when Python is configured with the `--with-fpectl' option, or the `WANT_SIGFPE_HANDLER' symbol is defined in the `config.h' file. `IOError' Raised when an I/O operation (such as a `print' statement, the built-in `open()' function or a method of a file object) fails for an I/O-related reason, e.g., "file not found" or "disk full". This class is derived from `EnvironmentError'. See the discussion above for more information on exception instance attributes. `ImportError' Raised when an `import' statement fails to find the module definition or when a `from ...import' fails to find a name that is to be imported. `IndexError' Raised when a sequence subscript is out of range. (Slice indices are silently truncated to fall in the allowed range; if an index is not a plain integer, `TypeError' is raised.) `KeyError' Raised when a mapping (dictionary) key is not found in the set of existing keys. `KeyboardInterrupt' Raised when the user hits the interrupt key (normally or ). During execution, a check for interrupts is made regularly. Interrupts typed when a built-in function `input()' or `raw_input()') is waiting for input also raise this exception. `MemoryError' Raised when an operation runs out of memory but the situation may still be rescued (by deleting some objects). The associated value is a string indicating what kind of (internal) operation ran out of memory. Note that because of the underlying memory management architecture (C's `malloc()' function), the interpreter may not always be able to completely recover from this situation; it nevertheless raises an exception so that a stack traceback can be printed, in case a run-away program was the cause. `NameError' Raised when a local or global name is not found. This applies only to unqualified names. The associated value is the name that could not be found. `NotImplementedError' This exception is derived from `RuntimeError'. In user defined base classes, abstract methods should raise this exception when they require derived classes to override the method. _Added in Python version 1.5.2_ `OSError' This class is derived from `EnvironmentError' and is used primarily as the `os' module's `os.error' exception. See `EnvironmentError' above for a description of the possible associated values. _Added in Python version 1.5.2_ `OverflowError' Raised when the result of an arithmetic operation is too large to be represented. This cannot occur for long integers (which would rather raise `MemoryError' than give up). Because of the lack of standardization of floating point exception handling in C, most floating point operations also aren't checked. For plain integers, all operations that can overflow are checked except left shift, where typical applications prefer to drop bits than raise an exception. `RuntimeError' Raised when an error is detected that doesn't fall in any of the other categories. The associated value is a string indicating what precisely went wrong. (This exception is mostly a relic from a previous version of the interpreter; it is not used very much any more.) `SyntaxError' Raised when the parser encounters a syntax error. This may occur in an `import' statement, in an `exec' statement, in a call to the built-in function `eval()' or `input()', or when reading the initial script or standard input (also interactively). When class exceptions are used, instances of this class have atttributes `filename', `lineno', `offset' and `text' for easier access to the details; for string exceptions, the associated value is usually a tuple of the form `(message, (filename, lineno, offset, text))'. For class exceptions, `str()' returns only the message. `SystemError' Raised when the interpreter finds an internal error, but the situation does not look so serious to cause it to abandon all hope. The associated value is a string indicating what went wrong (in low-level terms). You should report this to the author or maintainer of your Python interpreter. Be sure to report the version string of the Python interpreter (`sys.version'; it is also printed at the start of an interactive Python session), the exact error message (the exception's associated value) and if possible the source of the program that triggered the error. `SystemExit' This exception is raised by the `sys.exit()' function. When it is not handled, the Python interpreter exits; no stack traceback is printed. If the associated value is a plain integer, it specifies the system exit status (passed to C's `exit()' function); if it is `None', the exit status is zero; if it has another type (such as a string), the object's value is printed and the exit status is one. Instances have an attribute `code' which is set to the proposed exit status or error message (defaulting to `None'). Also, this exception derives directly from `Exception' and not `StandardError', since it is not technically an error. A call to `sys.exit()' is translated into an exception so that clean-up handlers (`finally' clauses of `try' statements) can be executed, and so that a debugger can execute a script without running the risk of losing control. The `os._exit()' function can be used if it is absolutely positively necessary to exit immediately (e.g., after a `fork()' in the child process). `TypeError' Raised when a built-in operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch. `UnboundLocalError' Raised when a reference is made to a local variable in a function or method, but no value has been bound to that variable. This is a subclass of `NameError'. _Added in Python version 2.0_ `UnicodeError' Raised when a Unicode-related encoding or decoding error occurs. It is a subclass of `ValueError'. _Added in Python version 2.0_ `ValueError' Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as `IndexError'. `WindowsError' Raised when a Windows-specific error occurs or when the error number does not correspond to an `errno' value. The `errno' and `strerror' values are created from the return values of the `GetLastError()' and `FormatMessage()' functions from the Windows Platform API. This is a subclass of `OSError'. _Added in Python version 2.0_ `ZeroDivisionError' Raised when the second argument of a division or modulo operation is zero. The associated value is a string indicating the type of the operands and the operation. The following exceptions are used as warning categories; see the `warnings' module for more information. `Warning' Base class for warning categories. `UserWarning' Base class for warnings generated by user code. `DeprecationWarning' Base class for warnings about deprecated features. `SyntaxWarning' Base class for warnings about dubious syntax `RuntimeWarning' Base class for warnings about dubious runtime behavior.