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I'm rusty on my Python, but I'd have to guess that no, Python's dynamic features don't get you the kind of power that Pascal's talking about. The key thing to

I'm rusty on my Python, but I'd have to guess that no, Python's dynamic features don't get you the kind of power that Pascal's talking about. The key thing to realize is that any particular feature that your language has doesn't necessarily matter. For example, Python may happend to have a built-in way of doing (with-input-file (in "myfile.txt") (do-something in)) In Python, this might look like withInputFile in "myfile.txt": doSomething(in) (hey, I told you I was rusty on my Python...especially my totally made up Python!) So maybe the feature came built-in to Python, in which case you're lucky--Guido happened to decide to implement this feature. But what if he hadn't? Well, just write your own abstraction. Obviously, Python can open files and do stuff with the file's contents. So it seems logical that you should be able to create an abstraction which saves you from writing the same boilerplate that you do every time you open an input file. Unfortunately, there are some abstractions that *you just can't write* in most languages (including Python). This file open example is a good one. Here's another example, in the form of a programming challenge: In Python, create an abstraction called "debugExpr" that consumes an expression E, and then does two things: 1) prints "Expression E = V", where E is the given expression, and V is the value E evaluates to. 2) returns the value V. For example, %python% 6 * debugExpr(2+3) Expression 2+3 = 5 30 I'm betting you can't implement debugExpr in Python. So assuming Guido didn't extend the language to implement this feature, you're pretty much SOL. It's a shame because debugExpr, I think, is pretty darn useful. It means that you can take any expression in your code and "inspect it", without worrying about adjusting the structure of your program, and without affecting its semantics. Without debugExpr, I'd have to change my example code to be something like this to get the same result: print "Expression 2+3 =" tmp = 2+3 print tmp 6 * tmp YUCK!

gms 20y ago
Well actually Python does have an eval() that can be called on expressions in the form of strings. The problem is that any potential macro anything would be nowhere near as seamless as it in Lisp, since the parse tree isn't right there for everything. But with enough effort (and closing your eyes to avoid how horrible it would look), you could maybe make something.
- vegashacker 20y ago
  I'm not sure. I think scoping would screw you up. First off, there's a small reason right away why it wouldn't work. You'd have to quote your argument to debugExpr, as in, say, debugExpr('2 times 3'). I did a little test at the Python prompt and the results didn't look good for faking this macro with eval: def foo(x): return 2 times (x + 3) > foo(1) 8 (not a full version of debugExpr, but enough to test:) def debugExpr(expr): print "Expr is " + expr return eval(expr) def bar(x): return 2 times debugExpr('x + 3') > bar(1) Expr is x + 3 Traceback (most recent call last): File "", line 1, in ? File "", line 2, in bar File "", line 3, in debugExpr File "", line 0, in ? NameError: name 'x' is not defined PS: I couldn't use the asterisk symbol in the post, so I used 'times' instead.

gms 20y ago

Well actually Python does have an eval() that can be called on expressions in the form of strings. The problem is that any potential macro anything would be nowhere near as seamless as it in Lisp, since the parse tree isn't right there for everything. But with enough effort (and closing your eyes to avoid how horrible it would look), you could maybe make something.

vegashacker 20y ago
I'm not sure. I think scoping would screw you up. First off, there's a small reason right away why it wouldn't work. You'd have to quote your argument to debugExpr, as in, say, debugExpr('2 times 3'). I did a little test at the Python prompt and the results didn't look good for faking this macro with eval: def foo(x): return 2 times (x + 3) > foo(1) 8 (not a full version of debugExpr, but enough to test:) def debugExpr(expr): print "Expr is " + expr return eval(expr) def bar(x): return 2 times debugExpr('x + 3') > bar(1) Expr is x + 3 Traceback (most recent call last): File "", line 1, in ? File "", line 2, in bar File "", line 3, in debugExpr File "", line 0, in ? NameError: name 'x' is not defined PS: I couldn't use the asterisk symbol in the post, so I used 'times' instead.