Re: [Tutor] binary, ascii, steganography
"Steven D'Aprano" wrote
with how to get binary to text, so your suggestions are greatly
appreciated.
I'll get to it! (0001 = a, 0010 = b, ...)
>>> int('01101110', 2)
110
>>> chr(110)
'n'
And if you really want to use non standard character values you
could look at the string maketrans() function and string.translate()
method...
HTH,
--
Alan Gauld
Author of the Learn to Program web site
http://www.alan-g.me.uk/
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Re: [Tutor] Waiting until a thread ends
> > >> Only really glanced at this, but you seem to be checking only the last >>> thread *after* the loop? Surely you should be storing all the threads in a >>> list (or someplace) as you create them, and then check them all for liveness >>> and if so join them each in turn, to ensure you only print 'FINISHED' once >>> you've checked and confirmed that all the threads created have in fact >>> finished. >>> >>> Walter >>> >>> >> That makes absolute sense. Doh on my part! >> >> Thanks! >> >> > Just done a little more reading and came across this in an O'Reilly article > here http://www.oreillynet.com/onlamp/blog/2008/01/pymotw_threading.html > > Seems like an elegant way to accomplish a wait until all running threads > have finished. > > Using enumerate() to wait for all running threads: > > It is not necessary to retain an explicit handle to all of the daemon > threads you start in order to ensure they have completed before exiting the > main process. threading.enumerate()returns a list of active Thread instances. > The list includes the current thread, and since joining the current thread > is not allowed (it introduces a deadlock situation), we must check before > joining. > > In my continuing quest the find the best way of doing this I came across the following method: for thread in threading.enumerate(): if thread is not threading.currentThread(): thread.join() print 'FINISHED' In my newbie understanding, you can't join() the current thread, because it's the main thread (the one from which the others are called), join()ing it would lock the program up (it would never complete). The above only join()s a thread if it isn't the current thread, thus (hopefully) getting around this. Swapping my earlier stupid code for this seems to work as expected in my tests. Chris ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] lambda
I am trying to figure out where lambda functions can be useful. Has anyone used them in real world? >From my reading so far, I hear people claim that lambda can be a useful replacement for small functions. Most examples didn't make much sense to me. Why would anyone use a one liner anonymous function, if you never plan to use it elsewhere? You would then be implementing the logic directly in the line, isn't it?. Functions are useful if they plan to get called multiple times within your code. For example: add_one = lambda x: x + 1 In real world, why would I use a lambda for this. I would simply do: add_one = x + 1 Can you provide some useful use cases for lambda functions? ~ Ajit Deshpande ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
[Tutor] Need some clarification on this
>>>a=5 >>>b=5 >>>a == b True >>>a is b True My question is, why "a is b" is true. What I expected it to be is that, a and b are different things with same value. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Need some clarification on this
2011/3/19 Yaşar Arabacı > >>>a=5 > >>>b=5 > >>>a == b > True > >>>a is b > True > > My question is, why "a is b" is true. What I expected it to be is that, a > and b are different things with same value. > > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > True means not zero, not "". Any numeric value but 0 is true. so you have True and True. I think what you are thinking of would be a == b. Since a does not equal b, the result is False -- Joel Goldstick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Need some clarification on this
2011/3/19 Yaşar Arabacı > >>>a=5 > >>>b=5 > >>>a == b > True > >>>a is b > True > > My question is, why "a is b" is true. What I expected it to be is that, a > and b are different things with same value. > > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > http://mail.python.org/mailman/listinfo/tutor > Oops. I misread your post. I (why I don't know) thought I saw a = 5, b = 4. a and b are names. Python has an integer of 5 from a = 5, so it just refers to that same object with b. -- Joel Goldstick ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] lambda
On 19/03/11 14:44, Ajit Deshpande wrote: I am trying to figure out where lambda functions can be useful. Has anyone used them in real world? From my reading so far, I hear people claim that lambda can be a useful replacement for small functions. Most examples didn't make much sense to me. Why would anyone use a one liner anonymous function, if you never plan to use it elsewhere? You would then be implementing the logic directly in the line, isn't it?. Functions are useful if they plan to get called multiple times within your code. For example: add_one = lambda x: x + 1 In real world, why would I use a lambda for this. I would simply do: add_one = x + 1 Can you provide some useful use cases for lambda functions? ~ Ajit Deshpande Here's one from the python docs, reduce takes a function so technically the programmer only uses it once but the program calls it many times. Map is another that works similarly. Have a look into functional programming as that's the branch of computer science those two functions and lambdas tend to come from I believe. http://docs.python.org/library/functions.html?highlight=lambda#reduce HTH, Adam. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] lambda
On 3/19/2011 9:44 AM, Ajit Deshpande wrote: I am trying to figure out where lambda functions can be useful. Has anyone used them in real world? From my reading so far, I hear people claim that lambda can be a useful replacement for small functions. Most examples didn't make much sense to me. Why would anyone use a one liner anonymous function, if you never plan to use it elsewhere? You would then be implementing the logic directly in the line, isn't it?. Functions are useful if they plan to get called multiple times within your code. For example: add_one = lambda x: x + 1 In real world, why would I use a lambda for this. I would simply do: add_one = x + 1 Can you provide some useful use cases for lambda functions? One is the list sort method. someList.sort([cmp[, key[, reverse]]]) /"cmp/ specifies a custom comparison function of two arguments (list items) which should return a negative, zero or positive number depending on whether the first argument is considered smaller than, equal to, or larger than the second argument: cmp=lambda x,y: cmp(x.lower(), y.lower())." -- Bob Gailer 919-636-4239 Chapel Hill NC ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] lambda
In my Python Pipelines program I have the following
opts = {
'characters' : (lambda rec, tot: tot + len(rec), 0),
'words' : (lambda rec, tot: tot + len(rec.split()), 0),
'lines' : (lambda rec, tot: tot + 1, 0),
'minline' :(lambda rec, tot: min(len(rec), tot), 9),
'maxline' :(lambda rec, tot: max(len(rec), tot), 0)
}
The user specifies option(s) (the dictionary keys). Thje program then
creates a class instance for each option, passing the corresponding
function as a class initialization parameter:
for option in options:
func, initTotal = Count.opts.get(option , 0)
if func:
ctr = Count.CounterClass(func, spec, initTotal)
it made lots of sense to me to code these as lambdas.
The alternatve would have been 4 defs: (4 more lines of code and more
indirection in code reading).
def char(rec, tot): return tot + len(rec)
etc
--
Bob Gailer
919-636-4239
Chapel Hill NC
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Re: [Tutor] Need some clarification on this
2011/3/19 Yaşar Arabacı > > > >>>a=5 > >>>b=5 > >>>a == b > True > >>>a is b > True > > My question is, why "a is b" is true. What I expected it to be is that, a > and b are different things with same value. > Even stranger: >>> a = 10**10 >>> b = 10**10 >>> a == b True >>> a is b False >>> a = 5 >>> b = 5 >>> a == b True >>> a is b True In the general case, you're right: a and b point to two different objects, but there is also some kind of optimisation for small numbers, and as a result when a = 5 and b = 5, both point the same '5' object. Emmanuel Ruellan ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Need some clarification on this
On Saturday 2011 March 19 08:35, Emmanuel Ruellan wrote: > 2011/3/19 Yaşar Arabacı > > > >>>a=5 > > >>>b=5 > > >>>a == b > > > > True > > > > >>>a is b > > > > True > > > > My question is, why "a is b" is true. What I expected it to be is that, a > > and b are different things with same value. > > Even stranger: > >>> a = 10**10 > >>> b = 10**10 > >>> a == b > > True > > >>> a is b > > False > > >>> a = 5 > >>> b = 5 > >>> a == b > > True > > >>> a is b > > True > > In the general case, you're right: a and b point to two different objects, > but there is also some kind of optimisation for small numbers, and as a > result when a = 5 and b = 5, both point the same '5' object. > > Emmanuel Ruellan From http://docs.python.org/c-api/int.html The current implementation keeps an array of integer objects for all integers between -5 and 256, when you create an int in that range you actually just get back a reference to the existing object. -- I have seen the future and I am not in it. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Need some clarification on this
2011/3/19 Yaşar Arabacı : a=5 b=5 a == b > True a is b > True > > My question is, why "a is b" is true. What I expected it to be is that, a > and b are different things with same value. > It's an optimization thing. When you type "a=5," the python interpreter is obligated to give you an integer object with the value 5. However, it doesn't need to be a *new* object. If an interpreter happens to have an object like that lying around already, it is perfectly allowed to give you that one. This doesn't mess things up for anyone, because integers are immutable. You can't change them, so it's safe to give the same object to different people. Interpreters can make the same optimization for strings, if they like. It makes it more efficient (since they don't have to allocate a new object) without changing the semantics of the language. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Need some clarification on this
This is a special feature called interning in python. As of Python 2.6,
values of -5 to 105 are never cleared from memory for performance reasons.
This is applicable to integers only.
"==" is a value comparator, whereas "is" is a reference compartor.
Check this interesting extension to your code:
>>> a=5.0
>>> b=5.0
>>> a==b
True
>>> a is b
False
Because I used "5.0" instead of "5", the "is" operator is giving a different
result ("False")
~ Ajit Deshpande
2011/3/19 Yaşar Arabacı
> >>>a=5
> >>>b=5
> >>>a == b
> True
> >>>a is b
> True
>
> My question is, why "a is b" is true. What I expected it to be is that, a
> and b are different things with same value.
>
> ___
> Tutor maillist - Tutor@python.org
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>
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Re: [Tutor] Need some clarification on this
On 01/-10/-28163 02:59 PM, Joel Goldstick wrote: 2011/3/19 Yaşar Arabacı a=5 b=5 a == b True a is b True My question is, why "a is b" is true. What I expected it to be is that, a and b are different things with same value. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor Oops. I misread your post. I (why I don't know) thought I saw a = 5, b = 4. a and b are names. Python has an integer of 5 from a = 5, so it just refers to that same object with b. Sometimes a particular implementation of Python will reuse the same object, if it's immutable. So certain strings and certain integers may be reused, presumably to save some space. But you cannot count on it. If you tried the same thing with a value of 741, you'd probably get a different answer. It's not time-efficient for Python to go hunting through all the existing objects to find a match, so this optimization is done pretty narrowly. You seldom want to use 'is' on an integer or a string anyway. Use == unless it really matters, which is usually either a custom object, or a singleton like True. DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] lambda
On 01/-10/-28163 02:59 PM, Ajit Deshpande wrote: I am trying to figure out where lambda functions can be useful. Has anyone used them in real world? From my reading so far, I hear people claim that lambda can be a useful replacement for small functions. Most examples didn't make much sense to me. Why would anyone use a one liner anonymous function, if you never plan to use it elsewhere? You would then be implementing the logic directly in the line, isn't it?. Functions are useful if they plan to get called multiple times within your code. For example: add_one = lambda x: x + 1 In real world, why would I use a lambda for this. I would simply do: add_one = x + 1 Can you provide some useful use cases for lambda functions? ~ Ajit Deshpande The other comments are useful. Lambdas are usually used for callback functions. But I wanted to point out that your two add_one objects are not at all equivalent. The first one is a function, which can be called at a later time. The second one is probably an integer, depending of course on the type of x. To get the equivalent of the lambda function above, you'd use def add_one(x): return x+1 DaveA ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Waiting until a thread ends
> > > >> In my continuing quest the find the best way of doing this I came across >> the following method: >> >> for thread in threading.enumerate(): >> if thread is not threading.currentThread(): >> thread.join() >> print 'FINISHED' >> >> In my newbie understanding, you can't join() the current thread, because >> it's the main thread (the one from which the others are called), join()ing >> it would lock the program up (it would never complete). >> >> The above only join()s a thread if it isn't the current thread, thus >> (hopefully) getting around this. Swapping my earlier stupid code for this >> seems to work as expected in my tests. >> > > Thanks for the postbacks, it's been useful/interesting for me. > > Best, > > Walter > I'm really pleased that it was of some help to somebody else too. Kind Regards, Chris ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] lambda
"Ajit Deshpande" wrote I am trying to figure out where lambda functions can be useful. Has anyone used them in real world? Yers lots of people, all over the place. They are very useful things. Why would anyone use a one liner anonymous function, To pass as an argument to another function. GUI widgets for example frequently take a function as an argument so that when they are clicked they call the function. If the function is only doing something trivial a lambda is better than writing a small function just to pass to the GUI widget. Similarly functions like sort() often take functions as arguments so that you can customise the sort algorithm. Often these are just one liners defining how a comparison should work. lambda comes from the matth/computer science term lambda calculus and lambdas are at the very core of theoretical computing. Functional programming is a style closely aligned to computer science theory and as such makes much use of lambdas (Lisp does this too). You can read more on the use of Lambdas and functional programming in the Functional Programming topic of my tutorial. You can see lambdas in use in the GUI topic of my tutor. HTH, -- Alan Gauld Author of the Learn to Program web site http://www.alan-g.me.uk/ ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] lambda
Ajit Deshpande wrote: I am trying to figure out where lambda functions can be useful. Has anyone used them in real world? Of course. lambdas are especially useful for callback functions, which are especially common when doing GUI programming. From my reading so far, I hear people claim that lambda can be a useful replacement for small functions. Most examples didn't make much sense to me. Why would anyone use a one liner anonymous function, if you never plan to use it elsewhere? That's precisely the point -- why would you define a non-anonymous function that you only use once? def spam(x): return x + something register_handler(spam) # Provide a callback to some part of your app # never use spam again is better written as: register_handler(lambda x: x+something) You would then be implementing the logic directly in the line, isn't it?. Functions are useful if they plan to get called multiple times within your code. That's often the case, but not necessarily. For example: add_one = lambda x: x + 1 This defines a function "add_one". This is equivalent to writing: def add_one(x): return x+1 In real world, why would I use a lambda for this. I would simply do: add_one = x + 1 This takes the current value of x, adds one to it, and assigns it to the very badly named variable "add_one". Can you provide some useful use cases for lambda functions? Sorting with comparison or key functions. min or max with a key function. Callbacks. map reduce Solving numerical equations. There are probably others, but they're the obvious ones. Here's an example you can try: text = """Li Europan lingues es membres del sam familie. Lor separat existentie es un myth. Por scientie, musica, sport etc, litot Europa usa li sam vocabular. Li lingues differe solmen in li grammatica, li pronunciation e li plu commun vocabules. Omnicos directe al desirabilite de un nov lingua franca: On refusa continuar payar custosi traductores.""" words = text.lower().split() sorted(words) # sort in alphabetical order Now suppose you want to sort by the number of vowels. Here's how you can do it with an anonymous key function: sorted(words, key=lambda word: sum(word.count(c) for c in 'aeiou')) -- Steven ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: http://mail.python.org/mailman/listinfo/tutor
