Version 11.2 has a new import/export format PythonExpression.

Unfortunately, it doesn't work out-of-the-box with images:

enter image description here

And using the ImageData directly is too slow:

enter image description here

Anyhow, I have some image processing functionality in Python, my question is how can I use ExternalEvaluate to call my function on an image from within Mathematica?

Here's the naive approach (which clearly fails at ToString):

script = FileNameJoin[{$TemporaryDirectory, "functions.py"}];
Export[script, "
  from scipy import misc

  def process_image(img):
    new_img = img # process the image with python
    return misc.imresize(new_img, (20,20), interp='bilinear')

  ", "Text"];
session = StartExternalSession["Python"];
ExternalEvaluate[session, File[script]];

img = ExampleData[{"TestImage", "Mandrill"}];
ExternalEvaluate[session, "img = " <> ExportString[img, {"Base64", "PNG"}]] (*fixme*)
ExternalEvaluate[session, "process_image(img)"]

enter image description here

  • 1
    $\begingroup$ Try ExportString as PNG. Presumably scipy can import that data (I believe PIL can import from PNG string). $\endgroup$
    – b3m2a1
    Sep 19, 2017 at 0:48
  • $\begingroup$ @b3m2a1 I'm not sure that would be efficient or simple $\endgroup$
    – M.R.
    Sep 19, 2017 at 2:53
  • 1
    $\begingroup$ I mean you could export to file? But this is one of the major flaws Szabolcs identified with ExternalEvaluate. Can't pass these things in memory. $\endgroup$
    – b3m2a1
    Sep 19, 2017 at 2:55
  • 2
    $\begingroup$ This may be worth trying again with v12, there have been some upgrades to this area. $\endgroup$
    – Carl Lange
    Apr 26, 2019 at 8:50

1 Answer 1


Here I successfully received, decode and show the image with the help of io, PIL, base64 libraries (PIL should be installed):

img = ExampleData[{"TestImage", "House"}];

session = StartExternalSession["Python"];

  "import io;from PIL import Image;import base64;"];

 "img = " <> 
  ExportString[ExportString[img, {"Base64", "PNG"}], 
   "PythonExpression"] <> ";"] 

ExternalEvaluate[session, "image = Image.open(io.BytesIO(base64.b64decode(img)));image.show();"]


Because there is \n in ExportString[img, {"Base64", "PNG"}], you could also use ''' (triple quotes):

 "img = '''" <> ExportString[img, {"Base64", "PNG"}] <> "''';"]

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