[Python-ideas] Have REPL print less by default

Nick Coghlan ncoghlan at gmail.com
Tue Apr 19 09:12:16 EDT 2016

On 19 April 2016 at 14:52, Franklin? Lee <leewangzhong+python at gmail.com>

> On Mon, Apr 18, 2016 at 11:51 AM, Ethan Furman <ethan at stoneleaf.us> wrote:
> > Or even Nick's (that you snipped):
> >>
> >> Since the more the REPL does, the more opportunities there are for
> >
> >> it to break when debugging, having the output hooks be as simple as
> >> possible is quite desirable.
> I read this as, "It'll be more complicated, so it MIGHT be buggy."
> This seems solvable via design, and doesn't speak to whether it's
> good, in principle, to limit output per input. If you still think it's
> a good reason, then I probably didn't understand it correctly.

No, that's not what it means. It relates to one of the ways experienced
developers are able to debug code: by running it in their heads, and
comparing the result their brain computed with what actually happened. When
there's a discrepancy, either their expectation is wrong or the code is
wrong, and they need to investigate further to figure out which it is. When
folks say "Python fits my brain" this is often what they're talking about.

Implicit side effects from hidden code break that mental equivalence - it's
why effective uses of metaclasses, monkeypatching, and other techniques for
deliberately introducing changed implicit behaviour often also involve
introducing some kind of more local signal to help convey what is going on
(such as a naming convention, or ensuring the altered behaviour is used
consistently across the entire project).

The default REPL behaviour is appropriate for this "somewhat experienced
Pythonista tinkering with code to see how it behaves" use case - keeping
the results very close to what they would be if you typed the same line of
code into a text file and ran it that way. It's not necessarily the best
way to *learn* those equivalences, but that's also not what it's designed

IPython's REPL is tailored for a different audience - their primary
audience is research scientists, and they want to be able to better eyeball
calculation results, rather than lower level Python instance
representations. As a result, it's much cleverer than the default REPL, but
it's also aiming to tap into people's intuitions about the shape of their
data and the expected outcomes of the operations they're performing on it,
rather than their ability to mentally run Python code specifically.

A REPL designed specifically for folks learning Python, like the one in the
Mu editor, or the direction IDLE seems to be going, would likely be better
off choosing different default settings for sys displayhook and
sys.excepthook, but those changes would be best selected based on direct
observations of classrooms and workshops, and noting where folks get
confused or intimidated by the default settings. For environments other
than IDLE, they can also be iterated on at a much higher rate than we make
CPython releases.


Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia
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