Warning
This documentation is for an old version of IPython. You can find docs for newer versions here.
Execution semantics in the IPython kernel¶
The execution of use code consists of the following phases:
- Fire the
pre_execute
event. - Fire the
pre_run_cell
event unless silent is True. - Execute the
code
field, 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.
See also
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 tosys.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.