$ git clone https://github.com/RustPython/RustPython
$ cd RustPython
$ cargo run demo.py
Hello, RustPython!

$ cargo run
Welcome to rustpython
>>>>> 2+2
4

$ cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.09s
     Running `target/debug/rustpython`
thread 'main' panicked at 'initializiation failed; run with RUST_BACKTRACE=1 to see Python backtrace', vm/src/vm.rs:537:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

$ RUST_BACKTRACE=1 cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.07s
     Running `target/debug/rustpython`
ModuleNotFoundError: No module named 'io'
thread 'main' panicked at 'initializiation failed; exception backtrace above', vm/src/vm.rs:537:9
stack backtrace:
   0: backtrace::backtrace::libunwind::trace
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/libunwind.rs:88
   1: backtrace::backtrace::trace_unsynchronized
             at /cargo/registry/src/github.com-1ecc6299db9ec823/backtrace-0.3.40/src/backtrace/mod.rs:66
   2: std::sys_common::backtrace::_print_fmt
             at src/libstd/sys_common/backtrace.rs:77
   3: <std::sys_common::backtrace::_print::DisplayBacktrace as core::fmt::Display>::fmt
             at src/libstd/sys_common/backtrace.rs:59
   4: core::fmt::write
             at src/libcore/fmt/mod.rs:1052
   5: std::io::Write::write_fmt
             at src/libstd/io/mod.rs:1426
   6: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:62
   7: std::sys_common::backtrace::print
             at src/libstd/sys_common/backtrace.rs:49
   8: std::panicking::default_hook::{{closure}}
             at src/libstd/panicking.rs:204
   9: std::panicking::default_hook
             at src/libstd/panicking.rs:224
  10: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:472
  11: rust_begin_unwind
             at src/libstd/panicking.rs:380
  12: std::panicking::begin_panic_fmt
             at src/libstd/panicking.rs:334
  13: rustpython_vm::vm::VirtualMachine::_py_panic_failed
             at vm/src/vm.rs:537
  14: rustpython_vm::vm::VirtualMachine::expect_pyresult::{{closure}}
             at vm/src/vm.rs:545
  15: core::result::Result<T,E>::unwrap_or_else
             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libcore/result.rs:848
  16: rustpython_vm::vm::VirtualMachine::expect_pyresult
             at vm/src/vm.rs:545
  17: rustpython_vm::vm::VirtualMachine::initialize
             at vm/src/vm.rs:268
  18: rustpython_vm::vm::VirtualMachine::new
             at vm/src/vm.rs:217
  19: rustpython::main
             at src/main.rs:39
  20: std::rt::lang_start::{{closure}}
             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libstd/rt.rs:67
  21: std::rt::lang_start_internal::{{closure}}
             at src/libstd/rt.rs:52
  22: std::panicking::try::do_call
             at src/libstd/panicking.rs:305
  23: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:86
  24: std::panicking::try
             at src/libstd/panicking.rs:281
  25: std::panic::catch_unwind
             at src/libstd/panic.rs:394
  26: std::rt::lang_start_internal
             at src/libstd/rt.rs:51
  27: std::rt::lang_start
             at /rustc/b8cedc00407a4c56a3bda1ed605c6fc166655447/src/libstd/rt.rs:67
  28: main
  29: __libc_start_main
  30: _start
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.