Mailman 3 October 2011 - Python-ideas

Proporsal: Show an alert in traceback when the source file is newer than compiled code (issue8087)
by Diego Mascialino 10 Oct '11

10 Oct '11

Hello, I opened this issue a long time ago, and Éric Araujo said that it should be discussed on this list. This is my feature request message: Example: ---------------- mod.py ---------------- def f(): a,b,c = 1,2 print b ---------------------------------------- If i do: >>> import mod >>> mod.f() I get: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "mod.py", line 2, in f a,b,c = 1,2 ValueError: need more than 2 values to unpack If i fix the source: ---------------- mod.py ---------------- def f(): a,b,c = 1,2,3 print b ---------------------------------------- And do: >>> mod.f() I get: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "mod.py", line 2, in f a,b,c = 1,2,3 ValueError: need more than 2 values to unpack The problem is that the source shown is updated, but the executed code is old, because it wasn't reloaded. Feature request: If the source code shown was modified after import time and it wasn't reloaded, a warning message should be shown. Example: Traceback (most recent call last): File "<stdin>", line 1, in <module> File "mod.py", line 2, in f WARNING: Modified after import! a,b,c = 1,2,3 ValueError: need more than 2 values to unpack or something like that. Thanks, Diego Mascialino

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Support multiplication for sets
by Jakob Bowyer 10 Oct '11

10 Oct '11

Looking at this from a Math background, it seems that it would be nice for the set type to support multiplication. This would allow for the multiplication of sets to produce Cartesian products giving every single permutation. This would make set usage more intuitive for example; (assuming python3) a = set(["amy", "martin"]) b = set(["smith", "jones", "john"]) c = a * b print(c) set([('john', 'jones'), ('john', 'martin'), ('jones', 'john'), ('martin', 'amy'), ....]) This could be really easily achieved by giving a __mul__ method for sets. Currently trying to multiply sets gives a TypeError. Anyone got any views on this? Or am I barking up the wrong tree and saying something stupid.

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Testing Key-Value Membership In Dictionaries
by Karthick Sankarachary 10 Oct '11

10 Oct '11

Hello Python Ideas, Currently, to check whether a single key is in a dictionary, we use the "in" keyword. However, there is no built-in support for checking if a key-value pair belongs in a dictionary. Currently, we presuppose that the object being checked has the same type as that of the key. What if we allowed the "in" operator to accept a tuple that denotes a (mapped) key-value pair? Let us consider how that might work using the canonical example given in the tutorial: >>> tel = {'jack': 4098, 'sape': 4139} >>> ('jack', 4098) in tel True >>> ('jack', 4000) in tel False >>> 'jack' in tel True As you can see, the "in" operator would interpret the object as either a key or a key-value pair depending on the actual types of the object, key and value. In the key itself happens to be a tuple, then the key-value membership test would involve a nested tuple, whose first item is a tuple denoting the key. Best Regards, Karthick Sankarachary

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Re: [Python-ideas] [Python-Dev] PEP 3101 (Advanced string formatting) base 36 integer presentation type
by Victor Stinner 09 Oct '11

09 Oct '11

Le 08/10/2011 17:14, Victor Stinner a écrit : > Le 08/10/2011 15:03, Antoine Pitrou a écrit : >> On Fri, 07 Oct 2011 21:14:44 -0600 >> Jeffrey<jss.bulk(a)gmail.com> wrote: >>> I would like to suggest adding an integer presentation type for base 36 >>> to PEP 3101. I can't imagine that it would be a whole lot more >>> difficult than the existing types. Python's built-in long integers >>> provide a nice way to prototype and demonstrate cryptographic >>> operations, especially with asymmetric cryptography. (Alice and Bob >>> stuff.) Built-in functions provide modular reduction, modular >>> exponentiation, and lots of nice number theory stuff that supports a >>> variety of protocols and algorithms. A frequent need is to represent a >>> message by a number. Base 36 provides a way to represent all 26 letters >>> in a semi-standard way, and simple string transformations can >>> efficiently make zeros into spaces or vice versa. >> >> Why base 36 rather than, say, base 64 or even base 80? > > Base 85 is the most efficient base to format IPv6 addresses! > > http://tools.ietf.org/html/rfc1924 > > And Python doesn't provide builtin function for this base! > > Victor Oops, I answered to the wrong mailing list. Victor

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Default return values to int and float
by David Townshend 09 Oct '11

09 Oct '11

My idea is fairly simple: add a "default" argument to int and float, allowing a return value if the conversion fails. E.g: >>> float('cannot convert this', default=0.0) 0.0 I think there are many use cases for this, every time float() or int() are called with data that cannot be guaranteed to be numeric, it has to be checked and some sort of default behaviour applied. The above example is just much cleaner than: try: return float(s) except ValueError: return 0.0 Any takers? David

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back-references in comprehensions
by yoch melka 08 Oct '11

08 Oct '11

Hi, I would like to use backreferences in list comprehensions (or other comprehensions), such as : [[elt for elt in lst if elt] for lst in matrix if \{1}] # \{1} is back-reference to filter result of the # first comprehension ([elt for elt in lst if elt]) It would be possible to do this ? Thanks

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PEP 3101 (Advanced string formatting) base 36 integer presentation type
by Jeffrey 08 Oct '11

08 Oct '11

I would like to suggest adding an integer presentation type for base 36 to PEP 3101. I can't imagine that it would be a whole lot more difficult than the existing types. Python's built-in long integers provide a nice way to prototype and demonstrate cryptographic operations, especially with asymmetric cryptography. (Alice and Bob stuff.) Built-in functions provide modular reduction, modular exponentiation, and lots of nice number theory stuff that supports a variety of protocols and algorithms. A frequent need is to represent a message by a number. Base 36 provides a way to represent all 26 letters in a semi-standard way, and simple string transformations can efficiently make zeros into spaces or vice versa. long() can already take a radix argument of 36 to parse base 36 numbers. If a base 36 presentation type (say, 'z' or something) was available, it would be possible to base 36 numbers as a simple way to interpret a message as a number in number theoretic/asymmetric cryptographic applications. The standard answer (see, for example, the Wikipedia Base 36 article) is to code up a quick routine to loop around and generate each digit, but: - The kinds of numbers used in cryptographic applications are big, as in thousands of digits. Providing some sort of direct support would nicely facilitate computational efficiency as it would avoid going around an interpreted loop thousands of times for each conversion. - All the the examples I've seen on in a quick Google search use the horrible anti-idiom of building up a string by repeated concatenation. Obviously, doing it efficiently is not trivial. It would be nice to get it right and be done with it. - Yes, one could write his own routine to do it, but this would serve as an unnecessary distraction where the purpose of the code is to illustrate a cryptographic algorithm or protocol - Yes, one could write his own module, define his own type, and override everything, but it's a fair amount of work to cover everything off (longs are a part of the language's core syntax) and even then, you still have the clutter of manually invoking constructors to make your new type instead of the built-int type. It's also kind of a trivial thing to justify promulgating a standard module for, and without a standard module, it's one more preliminary that has to be done before you can get to the main algorithm. - "Batteries included" is kind of a nice philosophy. Is it practical to add a base36 integer presentation type (say, 'z' or 'Z' similar to hexadecimal's 'x' or 'X') to the existing integer presentation types list in PEP 3101 (or do I need to raise a brand new PEP for this near-triviality)? It would be a very parallel thing to the hexadecimal one which is doing almost all the same things. I can't imagine it breaking anything because it defines something new that previously wasn't defined as anything useful at all. Jeffrey <jss.bulk@@gmail.com <mailto:jeffrey.streifling@gmail.com>>

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A Protocol for Making an Object Immutable
by Eric Snow 07 Oct '11

07 Oct '11

As I was working on some code today, I found myself in a situation where I wanted to "freeze" an object. This topic has come up a few times since I've been following these lists, so I figured it wasn't too crazy an idea. Really quickly I hacked out a simple bit of code that I am using for what I needed, and later I posted it as a recipe on the ActiveState cookbook[1]. The gist of it is a __freeze__/__unfreeze__ protocol, supported respectively by two ABCs and two functions (freeze and unfreeze). I've come to learn that it's a good idea to do some digging in the list archives before posting an idea, and I'm glad I did. Apparently there is a nearly identical PEP (351) to what I was thinking of [2]. And just my luck, it was shot down pretty hard. So I have a couple questions. First, from what I've read so far, the idea of frozen/immutable seems to be tied pretty closely to being hashable. However, this wasn't applicable to the use case that I had. Instead, I just wanted to make sure certain attributes of a class of mine were effectively read-only. So, is being hashable necessarily tied to being immutable? Second, I realize that PEP 351 failed to pass muster, so I'm not holding my breath here. Shoot, I'm not even really proposing we add this protocol. Mostly, I found a simple model that worked for me and seemed more broadly applicable; and I wanted to see if it would be suitable for further research. Now, in light of PEP 351, I'm less convinced that it's so simple. Still, I've seen some ideas come back from the proverbial grave and wanted to see if this one is not quite dead, sir (or if I should avenge it <wink>). -eric [1] http://code.activestate.com/recipes/577895/ [2] http://www.python.org/dev/peps/pep-0351/

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PEP 335: Another use case
by Gerald Britton 05 Oct '11

05 Oct '11

>I've received the following comments concerning another>potential use case for PEP 335.>>-------- Original Message -------->Subject: PEP 335 and NumPy NA>Date: Tue, 4 Oct 2011 15:42:50 -0700>From: Mark Wiebe <mwwiebe at gmail.com>>To: Gregory Ewing <greg at cosc.canterbury.ac.nz>>>Hi Greg,>>I took a glance at the python-ideas list, and saw that you're proposing>an update to PEP 335. I recently did a short internship at Enthought>where they asked me to look into the "missing data" problem for NumPy,>and I believe the work from that provides further motivation for the>PEP. Basically, the approach to allow good handling of missing data is>emulating the R programming language by introducing a new value called>NA, similar to the built-in None. For an NA with a boolean type, you get>a three-valued logic, as described>in http://en.wikipedia.org/wiki/Three-valued_logic#Kleene_logic. In the>NA I added to NumPy, this truth table cannot be satisfied because of the>issue PEP 335 addresses: Interesting! Kinda like SQL, I think, which has True, False and Null (similar to NA in R). Weird to think SQL "solved" this problem 40 years ago! -- Gerald Britton

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PEP 335: Another use case
by Greg Ewing 05 Oct '11

05 Oct '11

I've received the following comments concerning another potential use case for PEP 335. -------- Original Message -------- Subject: PEP 335 and NumPy NA Date: Tue, 4 Oct 2011 15:42:50 -0700 From: Mark Wiebe <mwwiebe(a)gmail.com> To: Gregory Ewing <greg(a)cosc.canterbury.ac.nz> Hi Greg, I took a glance at the python-ideas list, and saw that you're proposing an update to PEP 335. I recently did a short internship at Enthought where they asked me to look into the "missing data" problem for NumPy, and I believe the work from that provides further motivation for the PEP. Basically, the approach to allow good handling of missing data is emulating the R programming language by introducing a new value called NA, similar to the built-in None. For an NA with a boolean type, you get a three-valued logic, as described in http://en.wikipedia.org/wiki/Three-valued_logic#Kleene_logic. In the NA I added to NumPy, this truth table cannot be satisfied because of the issue PEP 335 addresses: >>> import numpy as np >>> if True or np.NA: print "yes" ... yes >>> if np.NA or True: print "yes" ... Traceback (most recent call last): File "<stdin>", line 1, in <module> ValueError: numpy.NA represents an unknown missing value, so its truth value cannot be determined This can be worked around by using the bitwise operators, as mentioned in the PEP for the array case: >>> if True | np.NA: print "yes" ... yes >>> if np.NA | True: print "yes" ... yes Here are the documents with more information: https://github.com/numpy/numpy/blob/master/doc/source/reference/arrays.mask… https://github.com/numpy/numpy/blob/master/doc/neps/missing-data.rst Cheers, Mark

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