Python - how it saves on compile time

Python is interpreted every time you run the code ... or that's the simple first story you're told. But really it's not that simple ... and not that inefficient.

When you run a Python program, it is not interpreted line by line as you run it - it is interpreted all at once before it's run at all, and stored as "byte code" so that loops and methods don't need to be re-interpreted hundreds or thousands of times during a single execution ... the Python virtual machine (for that's what it is) simply comes back to the already-worked-out store.

It gets even more efficient when you come to modules that you load through from or import. Their virtual machine code is saves into files with the extension .pyc (or .pyo if you use the optimize option) and they are loaded directly without the need to recompile on future runs. A quick operating system check on file time stamps means that if you change your source file, it will be 'silently' recompiled and resaved.

Other languages which are described as 'scripting' languages use a similar technique. See here, for example, to see how Lua does it.
(written 2009-10-20)

Associated topics are indexed under

Y115 - Additional Python Facilities
  [4085] JSON from Python - first principles, easy example - (2013-05-13)
  [3469] Teaching dilemma - old tricks and techniques, or recent enhancements? - (2011-10-08)
  [3442] A demonstration of how many Python facilities work together - (2011-09-16)
  [3089] Python regular expressions - repeating, splitting, lookahead and lookbehind - (2010-12-17)
  [2790] Joining a MySQL table from within a Python program - (2010-06-02)
  [2786] Factory methods and SqLite in use in a Python teaching example - (2010-05-29)
  [2765] Running operating system commands from your Python program - (2010-05-14)
  [2764] Python decorators - your own, staticmethod and classmethod - (2010-05-14)
  [2746] Model - View - Controller demo, Sqlite - Python 3 - Qt4 - (2010-04-29)
  [2745] Connecting Python to sqlite and MySQL databases - (2010-04-28)
  [2721] Regular Expressions in Python - (2010-04-14)
  [2655] Python - what is going on around me? - (2010-02-28)
  [2435] Serialization - storing and reloading objects - (2009-10-04)
  [2407] Testing code in Python - doctest, unittest and others - (2009-09-16)
  [1876] Python Regular Expressions - (2008-11-08)
  [1337] A series of tyre damages - (2007-09-08)
  [1336] Ignore case in Regular Expression - (2007-09-08)
  [1305] Regular expressions made easy - building from components - (2007-08-16)
  [1149] Turning objects into something you can store - Pickling (Python) - (2007-04-15)
  [1136] Buffering output - why it is done and issues raised in Tcl, Perl, Python and PHP - (2007-04-06)
  [1043] Sending an email from Python - (2007-01-18)
  [901] Python - listing out the contents of all variables - (2006-10-21)
  [753] Python 3000 - the next generation - (2006-06-09)
  [672] Keeping your regular expressions simple - (2006-04-05)
  [663] Python to MySQL - (2006-03-31)
  [463] Splitting the difference - (2005-10-13)
  [239] What and why for the epoch - (2005-03-08)
  [208] Examples - Gadfly, NI Number, and Tcl to C interface - (2005-02-10)
  [183] The elegance of Python - (2005-01-19)

Y305 - Optimising Python
  [4088] Some tips and techniques for huge data handling in Python - (2013-05-15)
  [3766] Python timing - when to use a list, and when to use a generator - (2012-06-16)
  [2369] Using a cache for efficiency. Python and PHP examples - (2009-08-21)
  [2277] Python classes / courses - what version do we train on? - (2009-07-10)

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