[Python-Dev] str(container) should call str(item), not repr(item) (original) (raw)

Ondrej Certik ondrej at certik.cz
Mon Jul 28 10:28:34 CEST 2008

Hi,

as discussed before here:

http://mail.python.org/pipermail/python-3000/2008-May/013876.html

if you do:

from decimal import Decimal a = Decimal(40) print a, a2, a3 40 1600 64000 print [a, a2, a3] [Decimal("40"), Decimal("1600"), Decimal("64000")] print {a: 3} {Decimal("40"): 3} print {a: a**2} {Decimal("40"): Decimal("1600")}

i.e. the str on list (and tuple and dict) calls repr() on the elements, instead of str. This really seems to me like a bug. Because if I wanted the repr() representation, I'd call repr() on the list/tuple/dict. If I want a nice readable representation, I call str(). That's the philosophy, no?

I understant there can be technical reasons why this cannot be made into python 3.0, so why not 3.1? or 2.8?

If this cannot be made into python for some reason, how would you suggest us we solve the following problem in sympy (symbolic manipulation library):

from sympy import Symbol, srepr x = Symbol("x") l = [x, x2/2, x3/3, x**4/4] str(l) '[x, 1/2x**2, 1/3x3, 1/4*x4]' repr(l) '[x, 1/2x**2, 1/3x3, 1/4*x4]' srepr(l) "[Symbol('x'), Mul(Half(1, 2), Pow(Symbol('x'), Integer(2))), Mul(Rational(1, 3), Pow(Symbol('x'), Integer(3))), Mul(Rational(1, 4), Pow(Symbol('x'), Integer(4)))]"

As you can see, we currently have to hack our repr to return the same thing as str so that things look nice in lists and dictionaries. We provide the srepr() function that does what the repr should do instead. And as you can see, the result of srepr is really not very readable, but it is easy to convert it back to a sympy expression using eval. That's the purpose of repr(), no? However, you cannot easily convert the str() back to an expression, because the eval() doesn't know what "x" is. One solution that we'll probably use is that we'll change our repr() to the srepr output above, so str([x, x2,...]) will not be pretty and we'll use sys.displayhook to hook our own function into the interactive session that overcomes this python behavior. This solves the problem for interactive sessions, but it doesn't solve it if people just call "print [x, x2, ...]" inside their programs, for example when debugging. Another option is to write a C extension module that monkey patches the list class and changes the C pointer to the (currently null) str method to our own method. But I find it very hackish and also it requires a C module.

I was discussing this with other people at EuroSciPy2008 and everyone's first reaction that str(list) calls repr() on the elements was that it's a bug. So was my reaction when I discovered that. So, I think people find this highly unintuitive. So I just wanted to discuss this more with core Python developers what you think about it and if you also find this not very intuitive and if so, if there is any chance to get this fixed.

Thanks, Ondrej

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