iPython custom cell magic - store output in variable?

Question

I recently discovered iPython magic functions and wrote some custom magic. I would like to use cell magic to parse strings, change them slightly and return the result. Is there a way to store the output of my custom cell magic function in a variable?

I know you can store the output of a line magic function like this:

@register_line_magic
def linemagic(line, cell=None):
    #do something
    return line

hello = %linemagic hello
print(hello)

Which returns:

>>> hello

In case I have larger strings, I would like to use cell magic instead:

@register_cell_magic
def cellmagic(line, cell=None):
    #do something
    return cell

It's not possible to use hello = %%cellmagic ... to store the result. Is there another way to capture the function output?

Answer 1

You can use IPython's input/output caching system:

Output caching:

• _ (a single underscore): stores previous output, like Python’s default interpreter.
• __ (two underscores): next previous.
• ___ (three underscores): next-next previous.
• _n (n being the prompt counter): the result of output
• actually, _4, Out[4] or _oh[4] all do the same thing

Similarly, for Input caching:

• _i, _ii, _iii: store previous, next previous and next-next previous inputs.
• _i4, _ih[4] and In[4]: the content of input <n> (e.g 4)

In [2]: from IPython.core.magic import register_cell_magic

In [3]: @register_cell_magic
   ...: def cellmagic(line, cell=None):
   ...:     #do something
   ...:     return cell
   ...:

In [4]: %%cellmagic
   ...: "line0"
   ...: "line1"
   ...: "line2"
   ...:
   ...:
Out[4]: '"line0"\n"line1"\n"line2"\n\n'

In [5]: _
Out[5]: '"line0"\n"line1"\n"line2"\n\n'

In [6]: _4
Out[6]: '"line0"\n"line1"\n"line2"\n\n'

In [8]: _i4
Out[8]: '%%cellmagic\n"line0"\n"line1"\n"line2"'

In [9]: var = _4

In [10]: var
Out[10]: '"line0"\n"line1"\n"line2"\n\n'