Comments by "Lawrence D’Oliveiro" (@lawrencedoliveiro9104) on "The most controversial Python feature | Walrus operator" video.

  1. Probably to avoid the notorious C mistake of using an assignment when you meant to do an equality comparison.
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  2.  @Danny-sj6es  It cannot be used in any expression. Unlike in C, chained assignment is made a special syntactic case.
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  3.  @keris3920  But C++ doesn’t do λ-calculus, does it? Actually Python does a better job of implementing that than C++ does.
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  4.  @PythonPlusPlus  This works for me: return \ {kw : _kw_indexes for kw in keywords for _kw_indexes in (find_occurrences(document, kw),) }
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  5. I detect a PHP programmer ... or maybe Java. . . . . . VBA?
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  6. One’s a functional construct, the other is procedural.
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  7. What “definition” might you be referring to? https://en.wikipedia.org/wiki/Procedural_programming
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  8. You can change the behaviour of assignment when the LHS is an indexing operation or property access. E.g. a[b] = c gets interpreted as a.__setitem__(b, c) Properties are fun, because they depend on a Python concept called “descriptors”. I’m not sure C++ programmers can understand that. ;)
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  9. Python’s descriptors also make quite a few other things possible. For example, enums are implemented, not as a special built-in language feature as it is in C, C++ or Java, but just as a library module, written in Python itself. This also makes use of the metaclass feature.
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  10.  @tarashmmurmu8412  In Python 3.5 and later you have the “@” operator to use for the dot product.
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  11. One quite surprising use of operator overloads with custom Python types is in the “ctypes” standard library module. This is an extensive set of tools for creating “Pythonic” wrappers around shareable libraries that were not written with Python in mind. For example, the ctypes “c_int” type stands for an “int” in the underlying C code. This name can be used as a constructor to wrap a Python integer, to be passed to or returned from C code. But you can also write things like “c_int * 4” to represent an array of 4 ints. This expression returns a new type, which can itself be used as a constructor. Thus, the expression (ctypes.c_int * 4)(1, 2, 3, 4) allocates storage for holding 4 ints, initializes them to the given values, and returns a Python object representing that storage.
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  12.  @keris3920  C++ doesn’t have the “@” operator.
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  13. Another thing Python lets you overload is built-in functions like len() and abs(). These also dispatch to special methods within the object, for which you can provide custom implementations. For example, in Qahirah, my pure-Python wrapper for the Cairo 2D graphics library, I defined a Vector type, and defined abs(«vector») to return the magnitude of the vector.
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  14. Another clever point about defining custom indexing with ‌_‌_‌setitem‌_‌_‌ and ‌_‌_‌getitem‌_‌_‌ is that, combined with tuples and an interesting Python syntactic shortcut, they let you implement multidimensional arrays. Thus, instead of “a[(b, c)]”, where the index is the tuple “(b, c)”, you can leave off the parentheses and just write “a[b, c]”. And so on for any number of dimensions. Of course, this applies even if b and c are not numbers.
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  15.  @keris3920  In other words, you can’t really achieve all of that in C++.
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  16.  @keris3920  Don’t get me wrong, C++ has its place. But if Turing-completeness were all there was to it, we would all be programming ... Turing machines.
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  17.  @keris3920  Yes. And a Turing machine, while it works for a lot of mathematical proofs, is not a particularly practical way of carrying out real-world computations. So saying something is “Turing-complete” doesn’t mean you should use it for actual programming.
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  18. Another overloadable operator Python has, that C++ does not, is “in”, commonly used for membership tests.
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  19.  @keris3920  Yeah, it just slows down compiles instead. And it’s awkward to use, and it doesn’t get rid of having to fall back on the further awkwardness of #‌‌define and friends. Python can do everything you can do with your C++-Turing-completeness-at-compile-time, and do it more elegantly. For example, conditional definition of functions and classes: if «cond» : def func() : ... definition 1 for func ... #end func else : def func() : ... definition 2 for func ... #end func #end if where «cond» is the same kind of conditional expression you would write anywhere else.
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  20.  @keris3920  Python doesn’t need to do it at “compile time”, since the full language facilities are available at run time. So you use the same language to do dynamic construction of class and function objects as you would to make use of them, instead of having to resort to different (and clunkier) sub-languages for “compile time” operations. This is reflected in the relative complexities of the languages: the C++ spec is well over 1000 pages now, and continues to grow with every new revision, whereas the entire Python language definition is about an order of magnitude smaller. All the rest of Python’s capabilities come from library modules, building on the core language.
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  21.  @keris3920  If “C++ can do everything at runtime too”, then why does it need separate compile-time and preprocessor sublanguages?
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  22.  @keris3920  Well, you did say “C++ has several Turing complete subsets”. Without the preprocessor, you only have template programming left. What else is there? And adding more and more RTTI is just making C++ more and more like Python, isn’t it? Sure, low-level languages have their place. They implement the CPU-intensive stuff, in a form that a high-level language like Python can make use of. If you look, you see will see where Python is popular in such compute-intensive and data-intensive applications.
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  23.  @keris3920  Consider a basic λ-calculus expression like f = λx.λy.x + y Python accepts a straight transliteration of this: f = lambda x : lambda y : x + y Just to prove it works: add2 = f(2) add3 = f(3) and now the expression “add2(3)” returns 5, while “add3(4)” returns 7. That only works because Python functions are first-class objects, with lexical binding. C++ has never heard of lexical binding, has it?
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  24. Python Devs: “And this is how you implement the Y-operator from λ-calculus.” C++ Devs: unintelligible, angry swearing
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  25. If it never ends, then it becomes a habit.
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  26. Using “=” for assignment was a mistake. It was a mistake in Fortran, it was a mistake in C, it’s a mistake in all those languages that have copied C (including Python). Consider also the inconsistency: if “==” is the equality comparison, and negated versions of operators are just supposed to have “!” in front, then why isn’t the inequality comparison “!==”? The Algol language family (including Pascal and its offshoot Ada) invented “:=” for the assignment operator, reserving “=” for equality comparison. This I think makes much more sense. Perhaps walrus in Python was a backdoor way of introducing this usage, at least in part?
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  27.  @TheNewton  Just use “:=” for assignment, following the Algol tradition.
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  28. Look up POP-11. It has the concept of an operand stack, so assignments go naturally left-to-right. E.g. a, b -> a -> b swaps the values of “a” and “b”.
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  29. I have yet to find a use for this myself, so so far I have just been ignoring it. The “«true-part» if «cond» else «false-part»” conditional expression, on the other hand, gets on my nerves. Because Python needs a conditional-expression form, but that is just an abortion. So I came up with my own solution https://ldo17.tumblr.com/post/17544419704/conditional-expressions-in-python .
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  30. 2:22 I can’t think of any situation where I would use “with open(...)”. In CPython, the file object will be closed anyway once the last reference to it disappears. For read-only files, this behaviour is fine. For files I am writing to, I want to do an explicit flush or close at the end, for obvious reasons. Either way, a with-construct seems unnecessary or even gets in the way.
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  31. Can you give a non-contrived example of using it? Because I have yet to find one.
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  32.  @PythonPlusPlus  Why should it be? Python alllows one-element tuples precisely for situations like this.
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  33.  @PythonPlusPlus  Actually, I notice that you didn’t mention any need for “if” semantics, even though your code has an “if” in it. So which code makes more sense: the one with the spurious “if”, or the one which avoids it?
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  34.  @vogel2499  You don’t really mean that, do you?
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  35.  @PythonPlusPlus  Are you saying that loops must iterate at least twice in order to be proper loops?
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  36.  @PythonPlusPlus  So you have to decide beforehand how many times the loop is going to loop to decide whether to use a loop or not?
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  37.  @PythonPlusPlus  I figure computers are here to serve me, I am not here to serve them. They do what I want, not what they want, to solve my problems, not theirs. It is a quirk of Python that, while a “for”-statement does not introduce a new scope for the loop variable, a “for”-clause in a sequence comprehension does. As long as that feature exists, the walrus is completely unnecessary.
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