#+BEGIN_SRC python :results drawer
import matplotlib.pyplot as plt
plt.plot([1,2,3])
plt.show()
a=1
print("a={}".format{a})
#+END_SRC

does not fontify correctly in either python or ipython source blocks.

As a consequence, when exporting the org file to HTML (C-c C-e h h), it fails with the following message:

font-lock-fontify-keywords-region: Invalid function: #<subr progn>

If changing the code snippet to:

#+BEGIN_SRC python :results drawer
import matplotlib.pyplot as plt
plt.plot([1,2,3])
plt.show()
a=1
print("a=%s" % a)
#+END_SRC

the block fontifies and the file is exported to HTML correctly.

Can you reproduce this behaviour?

Many thanks in advance!