python philosophical question - strong vs duck typing

Terry Reedy tjreedy at
Wed Jan 4 01:37:03 EST 2012

On 1/3/2012 8:04 PM, Devin Jeanpierre wrote:

>> Can you give an actual example of such a definition of 'strongly typed
>> object' that excludes Python objects?
> I'd love to, but I neglected to above because I don't really know
> where we share ideas, and I wouldn't want to jump ahead and say
> something that seems outlandish. I hope I've done a decent enough job
> below. Stop me if I say something you don't agree with.
> Here's a short version: The term often refers to a continuum, where
> some languages are "stronger" than others, but there is no objective
> definition of "strong" -- it's like asking how big is "big", or how
> smart is "smart".

Fine so far. cold/hot, good/bad, etc are false dichotomies. Better to 
use there as comparatives -- better/worse that average, or than the 
middle 1/2. I don't use 'strong type' unless someone else does, and 
maybe I should stop being suckered into using it even then ;-).

 > The actual choice of what continuum strength refers
> to varies between people -- some specify it as "reinterpretation of
> bits in memory", in which case every dynamic language in the world is
> strongly typed (so I'll move on from that),


> or some as "implicit
> conversions between types", which is iffy because any function or
> overloaded operator can silently convert types (at least in Python),

Since Python does not 'silently convert types' as I understand those 3 
words, you lose me here. Can you give a code example of what you mean?

> meaning the language is as weak as the user (?). But they're both kind
> of reasonable and held by a few people, and it's easy to see how maybe
> somebody used to significantly more strict rules (OCaml, for example)
> might accuse Python of not being all that strong.  So you see it isn't
> really a change of definition, it's a redrawing of the lines on a
> continuum. The OCaml people are reasonable for saying what they say,
> because their point of reference is their favorite language, just as
> ours is ours. To us, maybe Haskell and OCaml are insanely
> strong/"strict", to them, we're insanely weak.

I am aware that typed names lead to typed function signatures and that 
some languages in the ML family even type operators, so that different 
operators are required for 1+2 and 1.0+2.0 and mixed operations like 
1+2.0 are prohibited. But we are talking about the typing of objects here.

> Here's another take, more in line with what I really meant: the
> continuums expressed above aren't universal either. IME they're chosen
> by dynamic typing advocates, which are worried about code readability
> and certain styles of immensely common errors that come from
> lower-level languages like C. Static typing advocates (at least in the
> functional programming language family) are instead concerned about
> correctness. For them, type systems are a way to reduce the number of
> errors in your code _in general_, not just the specific cases of type
> coercion or memory-safety. In this particular sense, Python is not
> very far from C

I see the object model of Python as being very different from the data 
model of C. The class of an object is an internal attribute of the 
object while the type of a block of bits is an external convention 
enforced by the compiler. I see Python's objects as more similar to the 
objects of many functional languages, except that Python has mutable 


> Type systems exist which are much "stronger" in what they can protect
> you against. As an extreme example, one might take a dependently-typed
> language. A statically dependently typed language can protect you
> against out-of-bounds array access with a proof at compile time. In

I am familiar with the length-parameterized array types of Pascal (which 
seem in practice to be a nuisance), but I see from
that there is a lot more to the subject.

> fact, a statically-dependently-typed language can do things like prove

sometimes, though not always,

> that two different implementations of the same function are in fact
> implementations of the same function -- this is neat for implementing
> a non-recursive version of a recursive function, and verifying the
> change doesn't influence semantics.

Using induction, I can prove, for instance, that these two functions

def f_tr(n, val=base): # tail recursive
     if n:
         return f_tr(n-1, rec(n, val))
         return val

def f_wi(n, val = base):  # while iterative
     while n:
         n, val = n-1, rec(n, val)
     return val

are equivalent, assuming enough stack and normal procedural Python 
semantics. (And assuming no typos ;-).

f_tr(0) == base == f_wi(0)
f_tr(n+1) == f_tr(n+1, base) == f_tr(n, rec(n+1, base))
== by inductive hypothsis
f_wi(n, rec(n+1, base)) == f_wi(n+1, base) == f_wi(n+1)

So it is not clear to me what such proofs have to do with types as I 
understand the term.

> It's definitely true that there is something here that Python's types
> cannot do, at least not without extremely heavily mangling
> (technically __instancecheck__ means that type objects are turing
> complete, but I request that we ignore that). These are the sorts of
> types that can, in a very real sense, replace unit tests. Just the
> fact that your program typechecks is proof that certain properties
> hold, and those properties can be very soothing, such as that example
> of proving two algorithms return the same output.

I can imagine that if one overloads 'type' with enough extra 
information, the procedural reasoning involved in such proofs might 
reduce to a more calculational demonstration. The question is whether 
the months and years of intellectual investment required on the front 
end pay off in reduced intellectual effort across several applications.

I happen to be working on a book on inductive algorithms that will 
include several such equivalences like that above. I will stick with the 
sort of proof that I can write and that most can understand.

> One last note about static vs dynamic vs strong typing: I do believe
> that a strongly typed dynamic+statically typed language is possible

Cython with superset features used.

> and a good thing. For statically typed code, run-time checks are
> disabled (speed),

I believe Cython disables the run-times checks that it can tell are 
unnecessary. For instance, "for i in range(len(array)): array[i]" does 
not need a runtime check that i is in range(len(array)) and hence can 
convert array[i] to its unchecked C equivalent.

  for dynamically typed code they are enabled, and all
> the while you can make relatively sophisticated queries about the type
> of a thing using an interactive interpreter (somewhat similar to
> unifying things in the prolog interpreter, in my view).

There is n

> So now that we've gotten this far, here's my proposition: It is
> reasonable to consider type systems in terms of how many errors they
> eliminate in your programs, rather than just readability/memory-safety

And interesting and for me, different viewpoint. I tend to be interested 
in correctness through understanding, or, if you will, logical 
correctness. Textual correctness, especially with what I do with Python, 
is a secondary issue.

Memory-safety does not directly concern me, since I am not writing or 
running servers. But it is a major target of malware.

To really eliminate errors, we need a multiplicity of types. We need not 
just floats, but non-negative floats, positive floats, and 'complex' 
floats that get promoted to complex as necessary. But the checks have to 
be dynamic, since in general, only constants can be statically checked 
at compile time. Or you need relative 'x is bigger than y' pair types so 
that x-y can be determined to be positive.

Hmmm. I guess that one could say that such a statement in the course of 
a proof amounts to a type statement. And the same for loop invariants.

> concerns. Also, it is reasonable to have relatively strict standards,
> if you're used to a very "strong" type system. Such a person might,
> very reasonably, decide that Python is quite weak.

Yes, only objects are (strongly ;-) typed. Names are untyped. Function 
parameters (signatures) are only directly typed as to whether the 
corresponding arguments are required or not and whether the matching of 
arguments and parameters is by position, name, or both. The 'type' of 
Python is something like "object-class based, procedural syntax, certain 
functional features". Strong/weak has nothing to do with Python itself.

> I admit that a lot of what I said here is putting words in other
> peoples' mouths, if you object, pretend I said it instead.

No objections except as noted.

 > I am, after all, a fan of ATS at the moment, and thus have some
 > interest in ridiculously strongly typed languages ;)

I see that in the list in the Wikipedia article.

> *phew*. That took a lot of time. I hope I haven't lost your attention.

No. I might even have learned something.

> [ An example of a simple dependently typed program:
> ]

Just got it after a minute delay.

Terry Jan Reedy

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