Python module os chmod file 664 does not change the permission to rw-rw-r-- but -w--wx----

Question

Recently I am using Python module os  when I tried to change the permission of a file  I did not get the expected result  For example  I intended to change the permission to rw-rw-r--   os chmod   tmp test file   664    The ownership permission is actually -w--wx---  230   --w--wx--- 1 ag ag 0 Mar 25 05 45 test file   However  if I change 664 to 0664 in the code  the result is just what I need  e g   os chmod   tmp test file   0664    The result is   -rw-rw-r-- 1 ag ag 0 Mar 25 05 55 test file   Could anybody help explaining why does that leading 0 is so important to get the correct result

User · Answer

leading 0 means this is octal constant  not the decimal one  and you need an octal to change file mode   permissions are a bit mask  for example  rwxrwx--- is 111111000 in binary  and it s very easy to group bits by 3 to convert to the octal  than calculate the decimal representation   0644  octal  is 0 110 100 100 in binary  i ve added dots for readability   or  as you may calculate  420 in decimal

User · Answer

Found this on a different forum     If you re wondering why that leading zero is important  it s because   permissions are set as an octal integer  and Python automagically   treats any integer with a leading zero as octal  So os chmod  file     484   in decimal  would give the same result    What you are doing is passing 664 which in octal is 1230  In your case you would need   os chmod   tmp test file   436       Update   Note  for Python 3 you have prefix with 0o  zero oh   E G  0o666

User · Answer

Use permission symbols instead of numbers  Your problem would have been avoided if you had used the more semantically named permission symbols rather than raw magic numbers  e g  for 664      usr bin env python3  import os import stat  os chmod       myfile       stat S IRUSR       stat S IWUSR       stat S IRGRP       stat S IWGRP       stat S IROTH     This is documented at https   docs python org 3 library os html os chmod and the names are the same as the POSIX C API values documented at man 2 stat   Another advantage is the greater portability as mentioned in the docs      Note  Although Windows supports chmod    you can only set the file   s read-only flag with it  via the stat S IWRITE and stat S IREAD constants or a corresponding integer value   All other bits are ignored    chmod  x is demonstrated at  How do you do a simple  quot chmod  x quot  from within python   Tested in Ubuntu 16 04  Python 3 5 2

User · Answer

Using the stat   bit masks does seem to me the most portable and explicit way of doing this  But on the other hand  I often forget how best to handle that  So  here s an example of masking out the  group  and  other  permissions and leaving  owner  permissions untouched  Using bitmasks and subtraction is a useful pattern   import os import stat def chmodme pn          Removes  group  and  other  perms  Doesn t touch  owner  perms         mode   os stat pn  st mode     mode -   mode  amp   stat S IRWXG   stat S IRWXO       os chmod pn  mode

User · Answer

If you have desired permissions saved to string then do  s    660  os chmod file path  int s  base 8

User · Answer

So for people who want semantics similar to     chmod 755 somefile   Use     python -c  import os  os chmod  somefile   0o755     If your Python is older than 2 6     python -c  import os  os chmod  somefile   0755

[python] Python module os.chmod(file, 664) does not change the permission to rw-rw-r-- but -w--wx----

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