Consider imports relative to their source

Currently they are considered relative to the current working directory
of the interpreter, which is incorrect.

Fixes RustPython#83.

Author: OddBloke

tests/test_snippets.py

@@ -54,7 +54,6 @@ def perform_test(filename, method, test_type):
 def run_via_cpython(filename):
     """ Simply invoke python itself on the script """
     env = os.environ.copy()
-    env['PYTHONPATH'] = '.'
     subprocess.check_call([sys.executable, filename], env=env)
 
 
@@ -78,7 +77,6 @@ def run_via_rustpython(filename, test_type):
     log_level = 'info' if test_type == _TestType.benchmark else 'trace'
     env['RUST_LOG'] = '{},cargo=error,jobserver=error'.format(log_level)
     env['RUST_BACKTRACE'] = '1'
-    env['PYTHONPATH'] = os.path.dirname(filename)
     subprocess.check_call(
         ['cargo', 'run', '--release', filename], env=env)
 

vm/src/import.rs

@@ -70,7 +70,7 @@ pub fn import(vm: &mut VirtualMachine, module: &String, symbol: &Option<String>)
 
 fn find_source(vm: &VirtualMachine, name: &String) -> io::Result<PathBuf> {
     let sys_path = vm.sys_module.get_item("path").unwrap();
-    let paths: Vec<PathBuf> = match sys_path.borrow().kind {
+    let mut paths: Vec<PathBuf> = match sys_path.borrow().kind {
         PyObjectKind::List { ref elements } => elements
             .iter()
             .filter_map(|item| match item.borrow().kind {
@@ -80,6 +80,19 @@ fn find_source(vm: &VirtualMachine, name: &String) -> io::Result<PathBuf> {
         _ => panic!("sys.path unexpectedly not a list"),
     };
 
+    let source_path = &vm.current_frame().code.source_path;
+    paths.insert(
+        0,
+        match source_path {
+            Some(source_path) => {
+                let mut source_pathbuf = PathBuf::from(source_path);
+                source_pathbuf.pop();
+                source_pathbuf
+            }
+            None => PathBuf::from("."),
+        },
+    );
+
     let suffixes = [".py", "/__init__.py"];
     let mut filepaths = vec![];
     for path in paths {

vm/src/sysmodule.rs

@@ -8,13 +8,12 @@ use std::env;
 use super::pyobject::{DictProtocol, PyContext, PyObjectRef};
 
 pub fn mk_module(ctx: &PyContext) -> PyObjectRef {
-    let mut path_list = match env::var_os("PYTHONPATH") {
+    let path_list = match env::var_os("PYTHONPATH") {
         Some(paths) => env::split_paths(&paths)
             .map(|path| ctx.new_str(path.to_str().unwrap().to_string()))
             .collect(),
         None => vec![],
     };
-    path_list.insert(0, ctx.new_str("".to_string()));
     let path = ctx.new_list(path_list);
     let modules = ctx.new_dict();
     let sys_name = "sys".to_string();