12.4 Changes to Built-in Operations

• For file objects, f.read(0) and f.readline(0) now return an empty string rather than reading an unlimited number of bytes. For the latter, omit the argument altogether or pass a negative value.
• A new system variable, sys.platform, has been added. It specifies the current platform, e.g. sunos5 or linux1.
• The built-in functions input() and raw_input() now use the GNU readline library when it has been configured (formerly, only interactive input to the interpreter itself was read using GNU readline). The GNU readline library provides elaborate line editing and history. The Python debugger (pdb) is the first beneficiary of this change.
• Two new built-in functions, globals() and locals(), provide access to dictionaries containming current global and local variables, respectively. (These augment rather than replace vars(), which returns the current local variables when called without an argument, and a module's global variables when called with an argument of type module.)
• The built-in function compile() now takes a third possible value for the kind of code to be compiled: specifying 'single' generates code for a single interactive statement, which prints the output of expression statements that evaluate to something else than None.