.. _execution_semantics:

Execution of cells in the IPython kernel

When IPython kernel receives execute_request <https://jupyter-client.readthedocs.io/en/latest/messaging.html#execute>_ with user code, it processes the message in the following phases:

1. Fire the pre_execute event.
2. Fire the pre_run_cell event unless silent is True.
3. Execute run_cell method to preprocess code, compile and run it, 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.
6. Fire the post_run_cell event unless silent is True.

.. seealso::

:doc:`/config/callbacks`

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 :func:compile function and executed.

Python :func: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 :func:sys.displayhook for any expression in the block that returns a value. This means that a single statement can actually produce multiple calls to :func: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. :func:sys.displayhook is never implicitly called.

eval A single expression that returns a value. :func: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 block 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 block 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 block is also multiline), run all in 'exec' mode as a single unit.