Execution of cells in the IPython kernel

When IPython kernel receives execute_request with user code, it processes the message in the following phases:

• Fire the pre_execute event.

• Fire the pre_run_cell event unless silent is True.

• Execute run_cell method to preprocess code, compile and run it, see below for details.

• If execution succeeds, expressions in user_expressions are computed. This ensures that any error in the expressions don't affect the main code execution.

• Fire the post_execute event.

• Fire the post_run_cell event unless silent is True.

Running user code

First, the code cell is transformed to expand %magic and !system commands by IPython.core.inputtransformer2. Then expanded cell is compiled using standard Python compile function and executed.

Python compile function provides mode argument to select one of three ways of compiling code:

single

Valid for a single interactive statement (though the source can contain multiple lines, such as a for loop). When compiled in this mode, the generated bytecode contains special instructions that trigger the calling of sys.displayhook for any expression in the block that returns a value. This means that a single statement can actually produce multiple calls to sys.displayhook, if for example it contains a loop where each iteration computes an unassigned expression would generate 10 calls

for i in range(10):
    i**2

exec

An arbitrary amount of source code, this is how modules are compiled. sys.displayhook is never implicitly called.

eval

A single expression that returns a value. sys.displayhook is never implicitly called.

The transformed code is parsed into a single abstract syntax tree, and its top-level nodes are executed one after the other. Which nodes get to display their value is controlled by the InteractiveShell.ast_node_interactivity option (default: last_expr): nodes selected for display (by default, the last node, if it is an expression) are compiled in 'single' mode so that sys.displayhook is triggered, while all other nodes are compiled in 'exec' mode. This makes it easy to type simple expressions at the end of a cell to see computed values. Other accepted values for ast_node_interactivity are all, last, none and last_expr_or_assign.