Added equivalent of shell utility "which" for Python

Author: hannes-ucsc

src/bd2k/util/processes.py

@@ -0,0 +1,35 @@
+import os
+
+
+def which( name, path=None ):
+    """
+    Look for an executable file of the given name in the given list of directories,
+    or the directories listed in the PATH variable of the current environment. Roughly the
+    equivalent of the `which` program. Does not work on Windows.
+
+    :param executable_name: the name of the program
+
+    :type executable_name: str
+
+    :param path: an iterable of directory paths to consider or None if the PATH environment
+    variable should be used
+
+    :returns: an iterator yielding the full path to every occurrance of an executable file of the
+    given name in a directory on the given path or the PATH environment variable if no path was
+    passed
+
+    >>> next( which('ls') )
+    '/bin/ls'
+    >>> list( which('asdalskhvxjvkjhsdasdnbmfiewwewe') )
+    []
+    >>> list( which('ls', path=()) )
+    []
+    """
+    if path is None:
+        path = os.environ.get( 'PATH' )
+        if path is None: return
+        path = path.split( os.pathsep )
+    for bin_dir in path:
+        executable_path = os.path.join( bin_dir, name )
+        if os.access( executable_path, os.X_OK ):
+            yield executable_path