Effect of using sys path insert 0 path and sys path append when loading modules

Question

I was recently having a problem with a python ImportError  where the module was found when running on my local computer but not found on the CI server  I solved this problem by swapping sys path append path  in my script with sys path insert 0  path  where path is the string module location   Since this is my module and not an installed package  related question   why does the order of paths fix this problem

User · Answer

I m quite a beginner in Python and I found the answer of Anand was very good but quite complicated to me  so I try to reformulate    1  insert and append methods are not specific to sys path and as in other languages they add an item into a list or array and       append item  add item to the end of the list      insert n  item  inserts the item at the nth position in the list  0 at the beginning  1 after the first element  etc        2  As Anand said  python search the import files in each directory of the path in the order of the path  so       If you have no file name collisions  the order of the path has no impact      If you look after a function already defined in the path and you use append to add your path  you will not get your function but the predefined one     But I think that it is better to use append and not insert to not overload the standard behaviour of Python  and use non-ambiguous names for your files and methods

User · Answer

Because python checks in the directories in sequential order starting at the first directory in sys path list  till it find the  py file it was looking for   Ideally  the current directory or the directory of the script is the first always the first element in the list  unless you modify it  like you did  From documentation -      As initialized upon program startup  the first item of this list  path 0   is the directory containing the script that was used to invoke the Python interpreter  If the script directory is not available  e g  if the interpreter is invoked interactively or if the script is read from standard input   path 0  is the empty string  which directs Python to search modules in the current directory first  Notice that the script directory is inserted before the entries inserted as a result of PYTHONPATH    So  most probably  you had a  py file with the same name as the module you were trying to import from  in the current directory  where the script was being run from    Also  a thing to note about ImportErrors   lets say the import error says - ImportError  No module named main - it doesn t mean the main py is overwritten  no if that was overwritten we would not be having issues trying to read it  Its some module above this that got overwritten with a  py or some other file   Example -  My directory structure looks like -   - test     - shared          -   init   py          - phtest py   - testmain py   Now From testmain py   I call from shared import phtest   it works fine   Now lets say I introduce a shared py in test directory    example -   - test     - shared          -   init   py          - phtest py   - testmain py    - shared py   Now when I try to do from shared import phtest from testmain py   I will get the error -  ImportError  cannot import name  phtest    As you can see above  the file that is causing the issue is shared py   not phtest py

[python] Effect of using sys.path.insert(0, path) and sys.path(append) when loading modules

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