Popen with conflicting executable/path
popen, python, subprocess, windows
Solution
Search for a program is not trivial. I'd specify the full path to the convert.exe executable explicitly instead.
`subprocess` uses `CreateProcess` on Windows that looks in `system32` directory even before any other directory in `%PATH%`:
... If the file name does not contain an extension, `.exe` is appended. Therefore, if the file name extension is .com, this parameter must include the .com extension. If the file name ends in a period (.) with no extension, or if the file name contains a path, .exe is not appended. If the file name does not contain a directory path, the system searches for the executable file in the following sequence:
- The directory from which the application loaded.
- The current directory for the parent process.
- The 32-bit Windows system directory. Use the GetSystemDirectory function to get the path of this directory.
- The 16-bit Windows system directory. There is no function that obtains the path of this directory, but it is searched. The name of this directory is System.
- The Windows directory. Use the GetWindowsDirectory function to get the path of this directory.
- The directories that are listed in the PATH environment variable. Note that this function does not search the per-application path specified by the App Paths registry key. To include this per-application path in the search sequence, use the ShellExecute function.
Therefore `convert` is equivalent to `convert.exe` in this case. It first looks in a directory that contains `sys.executable` e.g., `C:\Python27`. Then in the current directory: where you started the Python script from. Then in `system32` where it finds `convert.exe` (filesystem utility, not imagemagick).
You could try to remove system32 directory from `os.environ['PATH']` it may(?) suppress checking it: `Popen(cmd, env=no_system32_environ)` but it is fragile (worse than the explicit path).
There is a related issue on Python bug tracker: "Subprocess picks the wrong executable on Windows."
`cmd.exe` (the shell) uses different algorithm. See How does Windows locate files input in the shell?
If you set `shell=True` then the search sequence for `convert` program:
- `convert` is not an internal shell command
- there is no explicit path, so the search continues
- search the current directory
- search each directory specified by the PATH environment variable, in the order listed
`%PATHEXT%` defines which file extensions are checked and in what order e.g., convert.com, convert.exe, convert.bat, convert.cmd if `%PATHEXT%` is `.com;.exe;.bat;.cmd`.
Problem
I'd like to call the "convert" utility from ImageMagick from my Python script using Popen, like so: ``` Popen(["convert", input_path, "-flop", output_file_path]) ``` (The above example simply reverses the image horizontally) The problem is that when I run the script in Windows, it mistakenly calls the `convert.exe` utility that ships with Windows to convert FAT partitions to NTFS! (located in \Windows\system32) Now, if I randomly open a command prompt at any directory other than system32, and type "convert", it correctly runs the ImageMagick executable. So, this implies that Popen is automatically looking in system32. How can I make it not look in system32, and run the correct executable?