# Execute Python code

Once you have a VM instance, you can execute python code by calling exec method.

• source, the python source code to be executed
• filename, the filename of the source code. This is used for error reporting
• mode, the compile mode. See below for details
• module, the module where the code will be executed. If nullptr, the code will be executed in the __main__ module

exec handles possible exceptions and returns a PyObject*. If the execution is not successful, e.g. a syntax error or a runtime exception, the return value will be nullptr.

There are also overloaded versions of exec and eval, which is useful for simple execution:

• PyObject* exec(Str source)
• PyObject* eval(Str source)

The mode parameter controls how the source code is compiled. There are 5 possible values:

• EXEC_MODE, this is the default mode. Just do normal execution.
• EVAL_MODE, this mode is used for evaluating a single expression. The source should be a single expression. It cannot contain any statements.
• REPL_MODE, this mode is used for REPL. It is similar to EXEC_MODE, but generates PRINT_EXPR opcode when necessary.
• CELL_MODE, this mode is designed for Jupyter like execution. It is similar to EXEC_MODE, but generates PRINT_EXPR opcode when necessary.
• JSON_MODE, this mode is used for JSON parsing. It is similar to EVAL_MODE, but uses a lexing rule designed for JSON.

In some cases, you may want to execute python code in a more fine-grained way. These two methods are provided for this purpose:

• CodeObject_ compile(Str source, Str filename, CompileMode mode, bool unknown_global_scope)
• PyObject* _exec(CodeObject_ co, PyObject* _module)

1. compile compiles the source code into a CodeObject_ instance. Leave unknown_global_scope to false if you don't know what it means.
2. _exec executes the CodeObject_ instance.

try{
    PyObject* result = vm->exec("123", "<eval>", EVAL_MODE);
    std::cout << py_cast<int>(vm, result);   // 123
}catch(Exception& e){
    // use e.summary() to get a summary of the exception
    std::cerr << e.summary() << std::endl;
}