How does Jupyter detect Matplotlib plots in a cell?

JupyterLab
1

I am sorry - I realize this is a very ignorant question - but how does Jupyter know to generate Matplotlib plots in a code cell?

I am currently working on a pull-request to the Reticulate R framework that needs a good and robust method to detect whether a particular code cell (“chunk”) has generated a Matplotlib plot, so it can call plt.show() suitably, and display the plot in rendered document. How does Jupyter do this?

Sorry again for my lack of knowledge - thanks for any pointers.

2

ipykernel (the default kernel) checks the output for certain patterns to recognize the file formats.

# constants for identifying png/jpeg data
PNG = b"\x89PNG\r\n\x1a\n"
# front of PNG base64-encoded
PNG64 = b"iVBORw0KG"
JPEG = b"\xff\xd8"
# front of JPEG base64-encoded
JPEG64 = b"/9"
# constants for identifying gif data
GIF_64 = b"R0lGODdh"
GIF89_64 = b"R0lGODlh"
# front of PDF base64-encoded
PDF64 = b"JVBER"

Copied from here: ipykernel/ipykernel/jsonutil.py at 04eb1b99b4f0387ab352cc1ad092eeaf7c49e001 · ipython/ipykernel · GitHub

3

Thanks for the reply! I couldn’t immediately see where those constants were used - grep just picked up a test. Do you know where and how they are deployed to do the work of detecting plot outputs?

4

no, but i’d test for the type of an output and if it is bytes check for known mime headers/footers.
maybe those constants aren’t used anymore and are just old code, maybe they are used by some other module.
i don’t know how jupyter/ipykernel does it but for the simple ipykernels i built i used exactly the same method.

5

Ah - but the problem is - I don’t have easy access to all the outputs. For example, if the code includes res = plt.plot(range(10)) - I’d have to go in, analyze the code, find the variable name, and inspect the result - and that in the simplest case. So I was hoping to do something like:

n_figures = len(get_figures())
run_the_code(the_code)
if len(get_figures() > n_figures):
    # Do something