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: 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. While traditionally, most exceptions have been string objects, in Python 1.5, all standard exceptions have been converted to class objects, and users are encouraged to do the same. The source code for those exceptions is present in the standard library module `exceptions'; this module never needs to be imported explicitly. For backward compatibility, when Python is invoked with the `-X' option, most of the standard exceptions are strings(1). This option may be used to run code that breaks because of the different semantics of class based exceptions. The `-X' option will become obsolete in future Python versions, so the recommended solution is to fix the code. 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. When string-based standard exceptions are used, they are tuples containing the directly derived classes. `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'. `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. They are class objects, except when the `-X' option is used to revert back to string-based standard exceptions. `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. When class exceptions are used, the instance has 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. `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'. `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. ---------- Footnotes ---------- (1) For forward-compatibility the new exceptions `Exception', `LookupError', `ArithmeticError', `EnvironmentError', and `StandardError' are tuples.  File: python-lib.info, Node: Built-in Functions, Prev: Built-in Exceptions, Up: Built-in Objects Built-in Functions ================== The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order. `__import__(name[, globals[, locals[, fromlist]]])' This function is invoked by the `import' statement. It mainly exists so that you can replace it with another function that has a compatible interface, in order to change the semantics of the `import' statement. For examples of why and how you would do this, see the standard library modules `ihooks' and `rexec'. See also the built-in module `imp', which defines some useful operations out of which you can build your own `__import__()' function. For example, the statement ``import' `spam'' results in the following call: `__import__('spam',' `globals(),' `locals(), [])'; the statement `from' `spam.ham import' `eggs' results in `__import__('spam.ham',' `globals(),' `locals(),' `['eggs'])'. Note that even though `locals()' and `['eggs']' are passed in as arguments, the `__import__()' function does not set the local variable named `eggs'; this is done by subsequent code that is generated for the import statement. (In fact, the standard implementation does not use its LOCALS argument at all, and uses its GLOBALS only to determine the package context of the `import' statement.) When the NAME variable is of the form `package.module', normally, the top-level package (the name up till the first dot) is returned, *not* the module named by NAME. However, when a non-empty FROMLIST argument is given, the module named by NAME is returned. This is done for compatibility with the bytecode generated for the different kinds of import statement; when using `import spam.ham.eggs', the top-level package `spam' must be placed in the importing namespace, but when using `from spam.ham import eggs', the `spam.ham' subpackage must be used to find the `eggs' variable. As a workaround for this behavior, use `getattr()' to extract the desired components. For example, you could define the following helper: import string def my_import(name): mod = __import__(name) components = string.split(name, '.') for comp in components[1:]: mod = getattr(mod, comp) return mod `abs(x)' Return the absolute value of a number. The argument may be a plain or long integer or a floating point number. If the argument is a complex number, its magnitude is returned. `apply(function, args[, keywords])' The FUNCTION argument must be a callable object (a user-defined or built-in function or method, or a class object) and the ARGS argument must be a sequence (if it is not a tuple, the sequence is first converted to a tuple). The FUNCTION is called with ARGS as the argument list; the number of arguments is the the length of the tuple. (This is different from just calling `FUNC(ARGS)', since in that case there is always exactly one argument.) If the optional KEYWORDS argument is present, it must be a dictionary whose keys are strings. It specifies keyword arguments to be added to the end of the the argument list. `buffer(object[, offset[, size]])' The OBJECT argument must be an object that supports the buffer call interface (such as strings, arrays, and buffers). A new buffer object will be created which references the OBJECT argument. The buffer object will be a slice from the beginning of OBJECT (or from the specified OFFSET). The slice will extend to the end of OBJECT (or will have a length given by the SIZE argument). `callable(object)' Return true if the OBJECT argument appears callable, false if not. If this returns true, it is still possible that a call fails, but if it is false, calling OBJECT will never succeed. Note that classes are callable (calling a class returns a new instance); class instances are callable if they have a `__call__()' method. `chr(i)' Return a string of one character whose ASCII code is the integer I, e.g., `chr(97)' returns the string `'a''. This is the inverse of `ord()'. The argument must be in the range [0..255], inclusive. `cmp(x, y)' Compare the two objects X and Y and return an integer according to the outcome. The return value is negative if `X < Y', zero if `X == Y' and strictly positive if `X > Y'. `coerce(x, y)' Return a tuple consisting of the two numeric arguments converted to a common type, using the same rules as used by arithmetic operations. `compile(string, filename, kind)' Compile the STRING into a code object. Code objects can be executed by an `exec' statement or evaluated by a call to `eval()'. The FILENAME argument should give the file from which the code was read; pass e.g. `''' if it wasn't read from a file. The KIND argument specifies what kind of code must be compiled; it can be `'exec'' if STRING consists of a sequence of statements, `'eval'' if it consists of a single expression, or `'single'' if it consists of a single interactive statement (in the latter case, expression statements that evaluate to something else than `None' will printed). `complex(real[, imag])' Create a complex number with the value REAL + IMAG*j or convert a string or number to a complex number. Each argument may be any numeric type (including complex). If IMAG is omitted, it defaults to zero and the function serves as a numeric conversion function like `int()', `long()' and `float()'; in this case it also accepts a string argument which should be a valid complex number. `delattr(object, name)' This is a relative of `setattr()'. The arguments are an object and a string. The string must be the name of one of the object's attributes. The function deletes the named attribute, provided the object allows it. For example, `delattr(X, 'FOOBAR')' is equivalent to `del X.FOOBAR'. `dir([object])' Without arguments, return the list of names in the current local symbol table. With an argument, attempts to return a list of valid attribute for that object. This information is gleaned from the object's `__dict__', `__methods__' and `__members__' attributes, if defined. The list is not necessarily complete; e.g., for classes, attributes defined in base classes are not included, and for class instances, methods are not included. The resulting list is sorted alphabetically. For example: >>> import sys >>> dir() ['sys'] >>> dir(sys) ['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout'] >>> `divmod(a, b)' Take two numbers as arguments and return a pair of numbers consisting of their quotient and remainder when using long division. With mixed operand types, the rules for binary arithmetic operators apply. For plain and long integers, the result is the same as `(A / B, A %{} B)'. For floating point numbers the result is `(Q, A %{} B)', where Q is usually `math.floor(A / B)' but may be 1 less than that. In any case `Q * B + A %{} B' is very close to A, if `A %{} B' is non-zero it has the same sign as B, and `0 <= abs(A %{} B) < abs(B)'. `eval(expression[, globals[, locals]])' The arguments are a string and two optional dictionaries. The EXPRESSION argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using the GLOBALS and LOCALS dictionaries as global and local name space. If the LOCALS dictionary is omitted it defaults to the GLOBALS dictionary. If both dictionaries are omitted, the expression is executed in the environment where `eval' is called. The return value is the result of the evaluated expression. Syntax errors are reported as exceptions. Example: >>> x = 1 >>> print eval('x+1') 2 This function can also be used to execute arbitrary code objects (e.g. created by `compile()'). In this case pass a code object instead of a string. The code object must have been compiled passing `'eval'' to the KIND argument. Hints: dynamic execution of statements is supported by the `exec' statement. Execution of statements from a file is supported by the `execfile()' function. The `globals()' and `locals()' functions returns the current global and local dictionary, respectively, which may be useful to pass around for use by `eval()' or `execfile()'. `execfile(file[, globals[, locals]])' This function is similar to the `exec' statement, but parses a file instead of a string. It is different from the `import' statement in that it does not use the module administration -- it reads the file unconditionally and does not create a new module.(1) The arguments are a file name and two optional dictionaries. The file is parsed and evaluated as a sequence of Python statements (similarly to a module) using the GLOBALS and LOCALS dictionaries as global and local name space. If the LOCALS dictionary is omitted it defaults to the GLOBALS dictionary. If both dictionaries are omitted, the expression is executed in the environment where `execfile()' is called. The return value is `None'. `filter(function, list)' Construct a list from those elements of LIST for which FUNCTION returns true. If LIST is a string or a tuple, the result also has that type; otherwise it is always a list. If FUNCTION is `None', the identity function is assumed, i.e. all elements of LIST that are false (zero or empty) are removed. `float(x)' Convert a string or a number to floating point. If the argument is a string, it must contain a possibly signed decimal or floating point number, possibly embedded in whitespace; this behaves identical to `string.atof(X)'. Otherwise, the argument may be a plain or long integer or a floating point number, and a floating point number with the same value (within Python's floating point precision) is returned. *Note:* When passing in a string, values for NaNand Infinity may be returned, depending on the underlying C library. The specific set of strings accepted which cause these values to be returned depends entirely on the C library and is known to vary. `getattr(object, name)' The arguments are an object and a string. The string must be the name of one of the object's attributes. The result is the value of that attribute. For example, `getattr(X, 'FOOBAR')' is equivalent to `X.FOOBAR'. `globals()' Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called). `hasattr(object, name)' The arguments are an object and a string. The result is 1 if the string is the name of one of the object's attributes, 0 if not. (This is implemented by calling `getattr(OBJECT, NAME)' and seeing whether it raises an exception or not.) `hash(object)' Return the hash value of the object (if it has one). Hash values are integers. They are used to quickly compare dictionary keys during a dictionary lookup. Numeric values that compare equal have the same hash value (even if they are of different types, e.g. 1 and 1.0). `hex(x)' Convert an integer number (of any size) to a hexadecimal string. The result is a valid Python expression. Note: this always yields an unsigned literal, e.g. on a 32-bit machine, `hex(-1)' yields `'0xffffffff''. When evaluated on a machine with the same word size, this literal is evaluated as -1; at a different word size, it may turn up as a large positive number or raise an `OverflowError' exception. `id(object)' Return the `identity' of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. (Two objects whose lifetimes are disjunct may have the same `id()' value.) (Implementation note: this is the address of the object.) `input([prompt])' Equivalent to `eval(raw_input(PROMPT))'. `intern(string)' Enter STRING in the table of "interned" strings and return the interned string - which is STRING itself or a copy. Interning strings is useful to gain a little performance on dictionary lookup - if the keys in a dictionary are interned, and the lookup key is interned, the key comparisons (after hashing) can be done by a pointer compare instead of a string compare. Normally, the names used in Python programs are automatically interned, and the dictionaries used to hold module, class or instance attributes have interned keys. Interned strings are immortal (i.e. never get garbage collected). `int(x)' Convert a string or number to a plain integer. If the argument is a string, it must contain a possibly signed decimal number representable as a Python integer, possibly embedded in whitespace; this behaves identical to `string.atoi(X)'. Otherwise, the argument may be a plain or long integer or a floating point number. Conversion of floating point numbers to integers is defined by the C semantics; normally the conversion truncates towards zero.(2) `isinstance(object, class)' Return true if the OBJECT argument is an instance of the CLASS argument, or of a (direct or indirect) subclass thereof. Also return true if CLASS is a type object and OBJECT is an object of that type. If OBJECT is not a class instance or a object of the given type, the function always returns false. If CLASS is neither a class object nor a type object, a `TypeError' exception is raised. `issubclass(class1, class2)' Return true if CLASS1 is a subclass (direct or indirect) of CLASS2. A class is considered a subclass of itself. If either argument is not a class object, a `TypeError' exception is raised. `len(s)' Return the length (the number of items) of an object. The argument may be a sequence (string, tuple or list) or a mapping (dictionary). `list(sequence)' Return a list whose items are the same and in the same order as SEQUENCE's items. If SEQUENCE is already a list, a copy is made and returned, similar to `SEQUENCE[:]'. For instance, `list('abc')' returns returns `['a', 'b', 'c']' and `list( (1, 2, 3) )' returns `[1, 2, 3]'. `locals()' Return a dictionary representing the current local symbol table. *Warning:* the contents of this dictionary should not be modified; changes may not affect the values of local variables used by the interpreter. `long(x)' Convert a string or number to a long integer. If the argument is a string, it must contain a possibly signed decimal number of arbitrary size, possibly embedded in whitespace; this behaves identical to `string.atol(X)'. Otherwise, the argument may be a plain or long integer or a floating point number, and a long integer with the same value is returned. Conversion of floating point numbers to integers is defined by the C semantics; see the description of `int()'. `map(function, list, ...)' Apply FUNCTION to every item of LIST and return a list of the results. If additional LIST arguments are passed, FUNCTION must take that many arguments and is applied to the items of all lists in parallel; if a list is shorter than another it is assumed to be extended with `None' items. If FUNCTION is `None', the identity function is assumed; if there are multiple list arguments, `map()' returns a list consisting of tuples containing the corresponding items from all lists (i.e. a kind of transpose operation). The LIST arguments may be any kind of sequence; the result is always a list. `max(s[, args...])' With a single argument S, return the largest item of a non-empty sequence (e.g., a string, tuple or list). With more than one argument, return the largest of the arguments. `min(s[, args...])' With a single argument S, return the smallest item of a non-empty sequence (e.g., a string, tuple or list). With more than one argument, return the smallest of the arguments. `oct(x)' Convert an integer number (of any size) to an octal string. The result is a valid Python expression. Note: this always yields an unsigned literal, e.g. on a 32-bit machine, `oct(-1)' yields `'037777777777''. When evaluated on a machine with the same word size, this literal is evaluated as -1; at a different word size, it may turn up as a large positive number or raise an `OverflowError' exception. `open(filename[, mode[, bufsize]])' Return a new file object (described earlier under Built-in Types). The first two arguments are the same as for `stdio''s `fopen()': FILENAME is the file name to be opened, MODE indicates how the file is to be opened: `'r'' for reading, `'w'' for writing (truncating an existing file), and `'a'' opens it for appending (which on *some* UNIX systems means that *all* writes append to the end of the file, regardless of the current seek position). Modes `'r+'', `'w+'' and `'a+'' open the file for updating (note that `'w+'' truncates the file). Append `'b'' to the mode to open the file in binary mode, on systems that differentiate between binary and text files (else it is ignored). If the file cannot be opened, `IOError' is raised. If MODE is omitted, it defaults to `'r''. When opening a binary file, you should append `'b'' to the MODE value for improved portability. (It's useful even on systems which don't treat binary and text files differently, where it serves as documentation.) The optional BUFSIZE argument specifies the file's desired buffer size: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of (approximately) that size. A negative BUFSIZE means to use the system default, which is usually line buffered for for tty devices and fully buffered for other files. If omitted, the system default is used.(3) `ord(c)' Return the ASCII value of a string of one character. E.g., `ord('a')' returns the integer `97'. This is the inverse of `chr()'. `pow(x, y[, z])' Return X to the power Y; if Z is present, return X to the power Y, modulo Z (computed more efficiently than `pow(X, Y) % Z'). The arguments must have numeric types. With mixed operand types, the rules for binary arithmetic operators apply. The effective operand type is also the type of the result; if the result is not expressible in this type, the function raises an exception; e.g., `pow(2, -1)' or `pow(2, 35000)' is not allowed. `range([start,] stop[, step])' This is a versatile function to create lists containing arithmetic progressions. It is most often used in `for' loops. The arguments must be plain integers. If the STEP argument is omitted, it defaults to `1'. If the START argument is omitted, it defaults to `0'. The full form returns a list of plain integers `[START, START + STEP, START + 2 * STEP, ...]'. If STEP is positive, the last element is the largest `START + I * STEP' less than STOP; if STEP is negative, the last element is the largest `START + I * STEP' greater than STOP. STEP must not be zero (or else `ValueError' is raised). Example: >>> range(10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] >>> range(1, 11) [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] >>> range(0, 30, 5) [0, 5, 10, 15, 20, 25] >>> range(0, 10, 3) [0, 3, 6, 9] >>> range(0, -10, -1) [0, -1, -2, -3, -4, -5, -6, -7, -8, -9] >>> range(0) [] >>> range(1, 0) [] >>> `raw_input([prompt])' If the PROMPT argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When `EOF' is read, `EOFError' is raised. Example: >>> s = raw_input('--> ') --> Monty Python's Flying Circus >>> s "Monty Python's Flying Circus" >>> If the `readline' module was loaded, then `raw_input()' will use it to provide elaborate line editing and history features. `reduce(function, sequence[, initializer])' Apply FUNCTION of two arguments cumulatively to the items of SEQUENCE, from left to right, so as to reduce the sequence to a single value. For example, `reduce(lambda x, y: x+y, [1, 2, 3, 4, 5])' calculates `((((1+2)+3)+4)+5)'. If the optional INITIALIZER is present, it is placed before the items of the sequence in the calculation, and serves as a default when the sequence is empty. `reload(module)' Re-parse and re-initialize an already imported MODULE. The argument must be a module object, so it must have been successfully imported before. This is useful if you have edited the module source file using an external editor and want to try out the new version without leaving the Python interpreter. The return value is the module object (i.e. the same as the MODULE argument). There are a number of caveats: If a module is syntactically correct but its initialization fails, the first `import' statement for it does not bind its name locally, but does store a (partially initialized) module object in `sys.modules'. To reload the module you must first `import' it again (this will bind the name to the partially initialized module object) before you can `reload()' it. When a module is reloaded, its dictionary (containing the module's global variables) is retained. Redefinitions of names will override the old definitions, so this is generally not a problem. If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module's advantage if it maintains a global table or cache of objects -- with a `try' statement it can test for the table's presence and skip its initialization if desired. It is legal though generally not very useful to reload built-in or dynamically loaded modules, except for `sys', `__main__' and `__builtin__'. In certain cases, however, extension modules are not designed to be initialized more than once, and may fail in arbitrary ways when reloaded. If a module imports objects from another module using `from' ... `import' ..., calling `reload()' for the other module does not redefine the objects imported from it -- one way around this is to re-execute the `from' statement, another is to use `import' and qualified names (MODULE.NAME) instead. If a module instantiates instances of a class, reloading the module that defines the class does not affect the method definitions of the instances -- they continue to use the old class definition. The same is true for derived classes. `repr(object)' Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to `eval()'. `round(x[, n])' Return the floating point value X rounded to N digits after the decimal point. If N is omitted, it defaults to zero. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minus N; if two multiples are equally close, rounding is done away from 0 (so e.g. `round(0.5)' is `1.0' and `round(-0.5)' is `-1.0'). `setattr(object, name, value)' This is the counterpart of `getattr()'. The arguments are an object, a string and an arbitrary value. The string may name an existing attribute or a new attribute. The function assigns the value to the attribute, provided the object allows it. For example, `setattr(X, 'FOOBAR', 123)' is equivalent to `X.FOOBAR = 123'. `slice([start,] stop[, step])' Return a slice object representing the set of indices specified by `range(START, STOP, STEP)'. The START and STEP arguments default to None. Slice objects have read-only data attributes `start', `stop' and `step' which merely return the argument values (or their default). They have no other explicit functionality; however they are used by Numerical Python and other third party extensions. Slice objects are also generated when extended indexing syntax is used, e.g. for `a[start:stop:step]' or `a[start:stop, i]'. `str(object)' Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference with `repr(OBJECT)' is that `str(OBJECT)' does not always attempt to return a string that is acceptable to `eval()'; its goal is to return a printable string. `tuple(sequence)' Return a tuple whose items are the same and in the same order as SEQUENCE's items. If SEQUENCE is already a tuple, it is returned unchanged. For instance, `tuple('abc')' returns returns `('a', 'b', 'c')' and `tuple([1, 2, 3])' returns `(1, 2, 3)'. `type(object)' Return the type of an OBJECT. The return value is a type object. The standard module `types' defines names for all built-in types. For instance: >>> import types >>> if type(x) == types.StringType: print "It's a string" `vars([object])' Without arguments, return a dictionary corresponding to the current local symbol table. With a module, class or class instance object as argument (or anything else that has a `__dict__' attribute), returns a dictionary corresponding to the object's symbol table. The returned dictionary should not be modified: the effects on the corresponding symbol table are undefined.(4) `xrange([start,] stop[, step])' This function is very similar to `range()', but returns an "xrange object" instead of a list. This is an opaque sequence type which yields the same values as the corresponding list, without actually storing them all simultaneously. The advantage of `xrange()' over `range()' is minimal (since `xrange()' still has to create the values when asked for them) except when a very large range is used on a memory-starved machine (e.g. MS-DOS) or when all of the range's elements are never used (e.g. when the loop is usually terminated with `break'). ---------- Footnotes ---------- (1) It is used relatively rarely so does not warrant being made into a statement. (2) This is ugly -- the language definition should require truncation towards zero. (3) Specifying a buffer size currently has no effect on systems that don't have `setvbuf()'. The interface to specify the buffer size is not done using a method that calls `setvbuf()', because that may dump core when called after any I/O has been performed, and there's no reliable way to determine whether this is the case. (4) In the current implementation, local variable bindings cannot normally be affected this way, but variables retrieved from other scopes (e.g. modules) can be. This may change.  File: python-lib.info, Node: Python Services, Next: String Services, Prev: Built-in Objects, Up: Top Python Services *************** The modules described in this chapter provide a wide range of services related to the Python interpreter and its interaction with its environment. Here's an overview: * Menu: * sys:: * types:: * UserDict:: * UserList:: * operator:: * traceback:: * linecache:: * pickle:: * cPickle:: * copy_reg:: * shelve:: * copy:: * marshal:: * imp:: * parser:: * symbol:: * token:: * keyword:: * tokenize:: * pyclbr:: * code:: * codeop:: * pprint:: * repr:: * py_compile:: * compileall:: * dis:: * new:: * site:: * user:: * __builtin__:: * __main__::