I don’t understand this python statement and specifically the : (colon) operator

c: d = 2+3

when I test with
c = False
d = 0

I see the following results
c = 5
d = 0

Why is c modified and d is not?

(Actual code I was looking at is line 61 & 62 of https://github.com/adobe/pdfservices-python-sdk-samples/blob/main/src/ocrpdf/ocr_pdf.py)