来自: Some Suggestions for Teaching Python | Curious Efficiency Nick Coghlan 2014-08-08 12:55

I recently had the chance to attend a Software Carpentry bootcamp at the University of Queensland (as a teaching assistant), as well as seeing a presentation from one of UQ’s tutors at PyCon Australia 2014.

While many of the issues they encountered were inherent in the complexity of teaching programming, a few seemed like things that could be avoided.

Getting floating point results from integer division

In Python 2, integer division copies C in truncating the answer by default:

$ python -c "print(3/4)"

Promoting to floating point requires type coercion, a command line flag or a future import:

$ python -c "print(float(3)/4)"
$ python -Qnew -c "print(3/4)"
$ python -c "from __future__ import division; print(3/4)"

Python 3 just does the right thing by default, so one way to avoid the problem entirely is to teach Python 3 instead of Python 2:

$ python3 -c "print(3/4)"

(In both Python 2 and 3, the // floor division operator explicitly requests truncating division when it is desired)

Common Python 2/3 syntax for printing values

I’ve been using Python 2 and 3 in parallel for more than 8 years now (while Python 3.0 was released in 2008, the project started in earnest a couple of years earlier than that, while Python 2.5 was still in development).

One essential trick I have learned in order to make regularly switching back and forth feasible is to limit myself to the common print syntax that works the same in both versions: passing a single argument surrounded by parentheses.

$ python -c 'print("Hello world!")'
Hello world!
$ python3 -c 'print("Hello world!")'
Hello world!

If I need to pass multiple arguments, I’ll use string formatting, rather than the implicit concatenation feature.

$ python -c 'print("{} {}{}".format("Hello", "world", "!"))'
Hello world!
$ python3 -c 'print("{} {}{}".format("Hello", "world", "!"))'
Hello world!

Rather than doing this, the Software Carpentry material that was used at the bootcamp I attended used the legacy Python 2 only print syntax extensively, causing examples that otherwise would have worked fine in either version to fail for students that happened to be running Python 3. Adopting the shared syntax for printing values could be enough to make the course largely version independent.

Distinguishing between returning and printing values

One problem noted both at the bootcamp and by presenters at PyCon Australia was the challenge of teaching students the difference between printing and returning values. The problem is the “Print” part of the Read-Eval-Print-Loop provided by Python’s interactive interpreter:

>>> def print_arg(x):
...     print(x)
>>> def return_arg(x):
...     return x
>>> print_arg(10)
>>> return_arg(10)

There’s no obvious difference in output at the interactive prompt, especially for types like numbers where the results of str and repr are the same. Even when they’re different, those differences may not be obvious to a student:

>>> print_arg("Hello world")
Hello world
>>> return_arg("Hello world")
'Hello world'

While I don’t have a definitive answer for this one, an experiment that seems worth trying to me is to teach students how to replace sys.displayhook. In particular, I suggest demonstrating the following change, and seeing if it helps explain the difference between printing output for display to the user and returning values for further processing:

>>> def new_displayhook(obj):
...     if obj is not None:
...         print("-> {!r}".format(obj))
>>> import sys
>>> sys.displayhook = new_displayhook
>>> print_arg(10)
>>> return_arg(10)
-> 10

Understanding the difference between printing and returning is essential to learning to use functions effectively, and tweaking the display of results this way may help make the difference more obvious.

Addendum: IPython (including IPython Notebook)

The initial examples above focused on the standard CPython runtime, include the default interactive interpreter. The IPython interactive interpreter, including the IPython Notebook, has a couple of interesting differences in behaviour that are relevant to the above comments.

Firstly, it does display return values and printed values differently, prefacing results with an output reference number:

In [1]: print 10

In [2]: 10
Out[2]: 10

Secondly, it has an optional “autocall” feature that allows a user to tell IPython to automatically add the missing parentheses to a function call if the user leaves them out:

$ ipython3 --autocall=1 -c "print 10"
-> print(10)

This is a general purpose feature that allows users to make their IPython sessions behave more like languages that don’t have first class functions (most notably, IPython’s autocall feature closely resembles MATLAB’s “command syntax” notation for calling functions).

It also has the side effect that users that use IPython, have autocall enabled, and don’t use any of the more esoteric quirks of the Python 2 print statement (like stream redirection or suppressing the trailing newline) may not even notice that print became an ordinary builtin in Python 3.


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