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 .