Execution semantics in the IPython kernel — IPython 4.0.0 documentation

# Execution semantics in the IPython kernel

The execution of use code consists of the following phases:

1. Fire the `pre_execute` event.
2. Fire the `pre_run_cell` event unless silent is True.
3. Execute the `code` field, see below for details.
4. If execution succeeds, expressions in `user_expressions` are computed. This ensures that any error in the expressions don’t affect the main code execution.
5. Fire the post_execute event.

See also

Registering callbacks

To understand how the `code` field is executed, one must know that Python code can be compiled in one of three modes (controlled by the `mode` argument to the `compile()` builtin):

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 `code` field is split into individual blocks each of which is valid for execution in ‘single’ mode, and then:

- If there is only a single block: it is executed in ‘single’ mode.
- If there is more than one block:
  - if the last one is a single line long, run all but the last in ‘exec’ mode and the very last one in ‘single’ mode. This makes it easy to type simple expressions at the end to see computed values.
  - if the last one is no more than two lines long, run all but the last in ‘exec’ mode and the very last one in ‘single’ mode. This makes it easy to type simple expressions at the end to see computed values. - otherwise (last one is also multiline), run all in ‘exec’ mode
  - otherwise (last one is also multiline), run all in ‘exec’ mode as a single unit.