The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order.
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.
>>> import sys >>> dir() ['sys'] >>> dir(sys) ['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout'] >>>
>>> 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().
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.
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'. 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. 2.10
>>> 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)  >>>
>>> 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.
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.
>>> import types >>> if type(x) == types.StringType: print "It's a string"