I have a python script that is under this directory:

```
work/project/test/a.py

```

Inside `a.py`, I use `subprocess.POPEN` to launch the process from another directory,

```
work/to_launch/file1.pl, file2.py, file3.py, ...

```

Python Code:

```
subprocess.POPEN("usr/bin/perl ../to_launch/file1.pl") 

```

and under work/project/, I type the following

```
[user@machine project]python test/a.py,

```

error "file2.py, 'No such file or directory'"

How can I add `work/to_launch/`, so that these dependent files `file2.py` can be found?