Where is Python s sys path initialized from

Question

Where is Python s sys path initialized from   UPD  Python is adding some paths before refering to PYTHONPATH        gt  gt  gt  import sys      gt  gt  gt  from pprint import pprint as p      gt  gt  gt  p sys path                 C   Python25  lib  site-packages  setuptools-0 6c9-py2 5 egg         C   Python25  lib  site-packages  orbited-0 7 8-py2 5 egg         C   Python25  lib  site-packages  morbid-0 8 6 1-py2 5 egg         C   Python25  lib  site-packages  demjson-1 4-py2 5 egg         C   Python25  lib  site-packages  stomper-0 2 2-py2 5 egg         C   Python25  lib  site-packages  uuid-1 30-py2 5 egg         C   Python25  lib  site-packages  stompservice-0 1 0-py2 5 egg         C   Python25  lib  site-packages  cherrypy-3 0 1-py2 5 egg         C   Python25  lib  site-packages  pyorbited-0 2 2-py2 5 egg         C   Python25  lib  site-packages  flup-1 0 1-py2 5 egg         C   Python25  lib  site-packages  wsgilog-0 1-py2 5 egg         c   testdir         C   Windows  system32  python25 zip         C   Python25  DLLs         C   Python25  lib         C   Python25  lib  plat-win         C   Python25  lib  lib-tk         C   Python25         C   Python25  lib  site-packages         C   Python25  lib  site-packages  PIL         C   Python25  lib  site-packages  win32         C   Python25  lib  site-packages  win32  lib         C   Python25  lib  site-packages  Pythonwin     My PYTHONPATH is       PYTHONPATH c  testdir   I wonder where those paths before PYTHONPATH s ones come from

User · Accepted Answer

Initialized from the environment variable PYTHONPATH  plus an installation-dependent default   -- http   docs python org library sys html sys path

User · Answer

Python really tries hard to intelligently set sys path  How it is set can get really complicated  The following guide is a watered-down  somewhat-incomplete  somewhat-wrong  but hopefully-useful guide for the rank-and-file python programmer of what happens when python figures out what to use as the initial values of sys path  sys executable  sys exec prefix  and sys prefix on a normal python installation   First  python does its level best to figure out its actual physical location on the filesystem based on what the operating system tells it  If the OS just says  python  is running  it finds itself in  PATH  It resolves any symbolic links  Once it has done this  the path of the executable that it finds is used as the value for sys executable  no ifs  ands  or buts   Next  it determines the initial values for sys exec prefix and  sys prefix   If there is a file called pyvenv cfg in the same directory as sys executable or one directory up  python looks at it  Different OSes do different things with this file   One of the values in this config file that python looks for is the configuration option home    lt DIRECTORY gt   Python will use this directory instead of the directory containing sys executable when it dynamically sets the initial value of sys prefix later  If the applocal   true setting appears in the pyvenv cfg file on Windows  but not the home    lt DIRECTORY gt  setting  then sys prefix will be set to the directory containing sys executable   Next  the PYTHONHOME environment variable is examined  On Linux and Mac  sys prefix and sys exec prefix are set to the PYTHONHOME environment variable  if it exists  superseding any home    lt DIRECTORY gt  setting in pyvenv cfg  On Windows  sys prefix and sys exec prefix is set to the PYTHONHOME environment variable  if it exists  unless a home    lt DIRECTORY gt  setting is present in pyvenv cfg  which is used instead   Otherwise  these sys prefix and sys exec prefix are found by walking backwards from the location of sys executable  or the home directory given by pyvenv cfg if any   If the file lib python lt version gt  dyn-load is found in that directory or any of its parent directories  that directory is set to be to be sys exec prefix on Linux or Mac  If the file lib python lt version gt  os py is is found in the directory or any of its subdirectories  that directory is set to be sys prefix on Linux  Mac  and Windows  with sys exec prefix set to the same value as sys prefix on Windows  This entire step is skipped on Windows if applocal   true is set  Either the directory of sys executable is used or  if home is set in pyvenv cfg  that is used instead for the initial value of sys prefix   If it can t find these  landmark  files or sys prefix hasn t been found yet  then python sets sys prefix to a  fallback  value  Linux and Mac  for example  use pre-compiled defaults as the values of sys prefix and sys exec prefix   Windows waits until sys path is fully figured out to set a fallback value for sys prefix   Then   what you ve all been waiting for   python determines the initial values that are to be contained in sys path    The directory of the script which python is executing is added to sys path  On Windows  this is always the empty string  which tells python to use the full path where the script is located instead  The contents of PYTHONPATH environment variable  if set  is added to sys path  unless you re on Windows and applocal is set to true in pyvenv cfg  The zip file path  which is  lt prefix gt  lib python35 zip on Linux Mac and os path join os dirname sys executable    python zip   on Windows  is added to sys path  If on Windows and no applocal   true was set in pyvenv cfg  then the contents of the subkeys of the registry key HK CURRENT USER Software Python PythonCore  lt DLLVersion gt  PythonPath  are added  if any  If on Windows and no applocal   true was set in pyvenv cfg  and sys prefix could not be found   then the core contents of the of the registry key HK CURRENT USER Software Python PythonCore  lt DLLVersion gt  PythonPath  is added  if it exists  If on Windows and no applocal   true was set in pyvenv cfg  then the contents of the subkeys of the registry key HK LOCAL MACHINE Software Python PythonCore  lt DLLVersion gt  PythonPath  are added  if any  If on Windows and no applocal   true was set in pyvenv cfg  and sys prefix could not be found   then the core contents of the of the registry key HK CURRENT USER Software Python PythonCore  lt DLLVersion gt  PythonPath  is added  if it exists  If on Windows  and PYTHONPATH was not set  the prefix was not found  and no registry keys were present  then the relative compile-time value of PYTHONPATH is added  otherwise  this step is ignored  Paths in the compile-time macro PYTHONPATH are added relative to the dynamically-found sys prefix  On Mac and Linux  the value of sys exec prefix is added  On Windows  the directory which was used  or would have been used  to search dynamically for sys prefix is added    At this stage on Windows  if no prefix was found  then python will try to determine it by searching all the directories in sys path for the landmark files  as it tried to do with the directory of sys executable previously  until it finds something  If it doesn t  sys prefix is left blank   Finally  after all this  Python loads the site module  which adds stuff yet further to sys path      It starts by constructing up to four directories from a head and a   tail part  For the head part  it uses sys prefix and sys exec prefix    empty heads are skipped  For the tail part  it uses the empty string   and then lib site-packages  on Windows  or lib pythonX Y site-packages   and then lib site-python  on Unix and Macintosh   For each of the   distinct head-tail combinations  it sees if it refers to an existing   directory  and if so  adds it to sys path and also inspects the newly   added path for configuration files

[python] Where is Python's sys.path initialized from?

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