[Tutor] who makes FOR loop quicker
To pass by reference or by copy of - that is the question from hamlet.
("hamlet" - a community of people smaller than a village python3.4-linux64)
xlist = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
i = 0
for x in xlist:
print(xlist)
print("\txlist[%d] = %d" % (i, x))
if x%2 == 0 :
xlist.remove(x)
print(xlist, "\n\n")
i = i + 1
So, catch the output and help, PLEASE, me improve the answer:
Does it appropriate ALWAYS reevaluate the terms of the list on each
iteration?
But if I want to pass a copy to FOR instead of a reference (as seen from
an output) and reduce unreasonable reevaluation, what I must to do for that?
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Re: [Tutor] who makes FOR loop quicker
Can You, please, elaborate this "..Passing in Python is different than
in C or other languages..."
'Cause as far as I know - default major Python's implementation CPython
is written in C.
Joel Goldstick 於 08/05/2015 03:44 PM 寫道:
On Wed, Aug 5, 2015 at 3:53 AM, John Doe wrote:
To pass by reference or by copy of - that is the question from hamlet.
("hamlet" - a community of people smaller than a village
python3.4-linux64)
xlist = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
i = 0
for x in xlist:
print(xlist)
print("\txlist[%d] = %d" % (i, x))
if x%2 == 0 :
xlist.remove(x)
print(xlist, "\n\n")
i = i + 1
So, catch the output and help, PLEASE, me improve the answer:
Does it appropriate ALWAYS reevaluate the terms of the list on each
iteration?
But if I want to pass a copy to FOR instead of a reference (as seen
from an
output) and reduce unreasonable reevaluation, what I must to do for
that?
You aren't passing anything. the for statement is in the same
namespace as the rest of the code. Passing in python is different
than in C or other languages.
A couple of comments:
setting i = 0, then incrementing at the end of the loop would more
pythonically be done with the enumerate function.
Its generally a bad idea to remove items from and iterable while
interating over it. I'm guessing that this is what is confusing you.
One way to remove items from a list is to create a new list, and
append items you want to it, skipping the ones you don't. You don't
really need the index at all since python interation protocol will
walk through the list for you without worrying about index values
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Re: [Tutor] who makes FOR loop quicker
Well, I think, both of us understands that any reference isn't about any sort of a language. It's about REGISTER = [ALU, FPU, ...] That's why reference inevitable. While You're talking about Python - You're talking ONLY about interpreter for a BYTEcode Alas, CPU don't speak BYTEcode but BITcode. So, Python can't allocate memory for CPU only for interpreter, which will ask allocation through underlying-C-language. Do I wrong? CPU have compiler for Python? As well as multithreading, for instance, in Python goes to a single interpreter, but in C - to multiple cores of CPU. So Python doesn't have REAL multithreading, but C - does. And in my case by means of C-rules Python allocates FOR-loop's list as a reference. And that mistake wastes each iteration of FOR-loop in unnecessary RE-evaluation of initial-list IN LOGIC STATEMENT, which must be created only once. Any INITIATIONS make once. 'Cause it sucks CPU-memory-allocation-cycle. Does this point make sense for You? Joel Goldstick 於 08/06/2015 03:57 PM 寫道: On Thu, Aug 6, 2015 at 4:34 AM, John Doe wrote: Can You, please, elaborate this "..Passing in Python is different than in C or other languages..." I hesitate, because this question is usually the fuel of flaming wars. So in short: C can pass a value or a reference to a value (the address of the place in memory where the value is stored) Python passes an object -- everything in python is an object. If the object is mutable, and the function mutates it, those results will be seen outside the function. If the object is immutable, and the function tries to change its value, a new object is created with the new value. Its name is the name given in the parameter list -- not the name that the function was called with. When the function completes, that object is lost since the outer scoped named object wasn't changed. 'Cause as far as I know - default major Python's implementation CPython is written in C. What language is used for its implementation has nothing to do with its own specification. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] who makes FOR loop quicker
Thank You, Steven. I've already written to Your colleague, so You will can see about. And when I'm saying 'ALLOCATION' I keep in mind the REGISTER, not a glossary or thesaurus. Language is created for us, not for CPU. Do You agree? Passing VALUE is a time-expensive procedure. Python can't reach processor, so any Python's memory allocations don't have sense. Language always was, is and will be the ONE - compiled bitcode. Others - just syntax + specially-sphere-applied variations for Your pleasure. Isn't it? Steven D'Aprano 於 08/06/2015 04:45 PM 寫道: On Thu, Aug 06, 2015 at 11:34:51AM +0300, John Doe wrote: Can You, please, elaborate this "..Passing in Python is different than in C or other languages..." Argument passing in Python is: - different to Perl, C, Scala, Algol and Pascal; - the same as Ruby, Lua, Applescript and Javascript; - the same as Java boxed values (object); - different to Java unboxed values (machine types). In C, all values are passed by value. When you pass an argument to a function, the C compiler makes a copy of that value and passes the value. In Pascal, values can be passed by value (like C), or by reference. The simplest demonstration of pass-by-reference is to write a "swap" procedure. In Python terms: # This does not actually work in Python. def swap(a, b): tmp = a a = b b = tmp x = 23 y = 42 swap(x, y) print x, y # prints 42 23 z = 19 swap(x, z) print x, z # prints 19 42 You *cannot* write a swap procedure like this in Python. The closest you can do is write a function that returns the two values, then assign them: def swap(a, b): return b, a x, y = swap(x, y) # this works but that is not pass by reference. In Pascal, you can write such a swap procedure. Scala and Algol use pass by name, and pass by value. This page explains pass by name in Scala, and how it differs from pass by value: http://alvinalexander.com/source-code/scala/simple-scala-call-name-example In Java, unboxed values (not objects, low-level machine ints and floats) are passed by value, like C. Python, Ruby, Javascript, Lua, Java boxed values (objects), and many other languages, all use the same passing style. This has a number of names: - pass by object; - pass by sharing; - pass by object sharing; Some people (especially Ruby programmers) call it "pass by reference" but that is wrong. Others (especially Java programmers) call it "call by value, where the value is a reference" which is technically correct but too long. Another name is "call by value/pass by reference", which is just confusing. See also: https://en.wikipedia.org/wiki/Evaluation_strategy In pass by object sharing, the argument is evaluated but *not* copied. Since the argument is not copied, it is not pass-by-value. Inside the function, you can modify the object, and since it is not a copy, the original sees the changes. But *assignment* to the local variable inside the function does not affect the caller's variable, so it is not pass-by-reference. To summarise: Pass by value: - Argument is copied? YES - Assignment inside function affects original? NO - Mutation of argument inside function affects original? NO Pass by reference: - Argument is copied? NO - Assignment inside function affects original? YES - Mutation of argument inside function affects original? YES Pass by object sharing: - Argument is copied? NO - Assignment inside function affects original? NO - Mutation of argument inside function affects original? YES 'Cause as far as I know - default major Python's implementation CPython is written in C. That is irrelevent. The argument passing strategy of a language is part of the language itself, not the implementation language. C does not allow variables to change type. But Python does. You cannot do this in C: x = 23 # x is an integer x = "foo" # and now it is a string so clearly the behaviour of a programming language is not always the same as that of the implementation language. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] who makes FOR loop quicker
Well... Try this look. But I'm just a human and can make mistakes.:)) Passing value - allocates stack and creates NEW memory position. Passing reference - makes stack pointer pointing to any position. Dereference - makes stack pointer pointing to any position AND TAKES VALUE. So, You can count how much in every case does CPU make steps. And now add to this BIG ARRAY as input for calculation. Always must keep in mind, that the NAME of variable exists for SCOPE of You code, but VALUE - for CPU. So, reference will be always, unless CPU have reached quantum-mechanics Steven D'Aprano 於 08/06/2015 05:21 PM 寫道: On Thu, Aug 06, 2015 at 08:57:34AM -0400, Joel Goldstick wrote: On Thu, Aug 6, 2015 at 4:34 AM, John Doe wrote: Can You, please, elaborate this "..Passing in Python is different than in C or other languages..." I hesitate, because this question is usually the fuel of flaming wars. Very wise :-) But since I'm not so wise, here are some more comments. So in short: C can pass a value or a reference to a value (the address of the place in memory where the value is stored) You are correct that C can pass a reference to a value, namely a pointer. But from the perspective of the C compiler, that pointer *is* the value, not the thing being pointed at. So passing a pointer as argument is no different from passing an int or a float or a bool, it's just a value, and the C compiler will use pass by value on the pointer itself. In C, one can use pointers to *simulate* pass by reference. But this is not the same thing as actual pass by reference. In pass by reference, you don't pass (a pointer to the variable you want), you pass (the variable you want), and the compiler does all the magic needed to make it work. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] who makes FOR loop quicker
Thank You, Alan. This is THE FIRST time, when I've got a pleasure from the opponent. You're maintain status of a thinking human and, as a humble DAOist, I always say THANK YOU, when I talk to such a Man. 'Cause wisdom bring us the beauty. So, what else I can add. Just a little bit. It would be great to work with You. You know, life is so shot.. And any talks don't make it better, alas. Just we can do. Alan Gauld 於 08/06/2015 09:54 PM 寫道: On 06/08/15 14:28, John Doe wrote: Well, I think, both of us understands that any reference isn't about any sort of a language. It's about REGISTER = [ALU, FPU, ...] No thats about the implementation. The language and implemewntation are completely searate. There can be many different implementations of a single language and they all have to follow the semantics defined by the language but are free to build those semantics any way they like. (And indeed the different versions of Python do just that!) While You're talking about Python - You're talking ONLY about interpreter for a BYTEcode Alas, CPU don't speak BYTEcode but BITcode. Some do. Some speak higher level things. For example some CPUs speak Forth. And still others don't use binary at all but use tri-state values. There werte even some in the early days that used 4-state values. But these are all implementation details that the programmer doesn't need to care about. So, Python can't allocate memory for CPU only for interpreter, which will ask allocation through underlying-C-language. Not necessarily. The interpreter could get C to allocate a huge pool of memory at startup and then use that for its own allocation/deallocation purposes. CPU have compiler for Python? Not yet but it is theoretically possible. And python would not change if someone built one. As well as multithreading, for instance, in Python goes to a single interpreter, but in C - to multiple cores of CPU. That depends on the implementation. I have a C compiler that does not do multi-core/thread working. It's an implementation detail and the C language does not specify that it must. It all depends on the code that the compiler generates. So Python doesn't have REAL multithreading, but C - does. Python does. But not all of its implementations do. The emulate it instead. But so far as the programmer is concerned his code is using threading/concurrency. He may need to be aware that the implementation is not honoring his request fully but that doesn't change his code. And in my case by means of C-rules Python allocates FOR-loop's list as a reference. No, the C implementation might do that. Python as a language does not. High level languages exist to stop us thinking about the low level details. The implementation may change, that's not our problem. Python specifies how the Python execution model works not how the CPU or the assembler, or the compiler or the implementer interprets that. Even C has many different implementations. Some are more efficient than others. Should we be worrying about which C compiler was used to build our interpreter? Should we care about whether the CPU implements multiplication in hardware or in microcode? Or whether it caches local variables on on-chip cache or uses main memory? And what about the I/O routines. Do we need to worry about whether our chosen C compiler is using it's own I/O library, or calling the BIOS directly? or using the OS system calls? These are all implementation details that regular programmers can, and should, ignore. And that mistake wastes each iteration of FOR-loop in unnecessary RE-evaluation of initial-list IN LOGIC STATEMENT, which must be created only once. Any INITIATIONS make once. 'Cause it sucks CPU-memory-allocation-cycle. In the modern world of fast CPUs and memory and where the vast majority of applications run in virtual machines(JVM, .Net) and the vast majority of servers run inside virtualized environments (VMWare etc) none of that is of the slightest concern to me. If I was writing code for an embedded system it might be more worrisome, but then I'd probably not be using Python for that. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[Tutor] Iterating Lines in File and Export Results
Hello List,
I am in need of your assistance. I have a text file with random words
in it. I want to write all the lines to a new file. Additionally, I am
using Python 2.7 on Ubuntu 12.04:
Here is my code:
def loop_extract():
with open('words.txt', 'r') as f:
for lines in f:
#print lines (I confirmed that each line is successfully printed)
with open('export.txt', 'w') as outf:
outf.write(lines)
#outf.write(lines)
#outf.write('{}\n'.format(lines))
#outf.write('{}\n'.format(line for line in lines))
For some reason, the second file only contains the last line from the
original file -- I have tried multiple variations (.read, .readlines,
.writelines, other examples preceded by comment from above and many
more) and tried to use the module, fileinput, but I still get the same
results.
I do understand there is another way to copy the file over, but to
provide additional background information on my purpose -- I want to
read a file and save successful regex matches to a file; exporting
specific data. There doesn't appear to be anything wrong with my
expression as it prints the expected results without failure. I then
decided to just write the export function by itself in its basic form,
per the code above, which the same behavior occurred; only copying the
last line. I've googled for hours and, unfortunately, at loss.
Thank you in advance for your help!
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Re: [Tutor] printing all text that begins with "25"
Hello, If you want to accomplish what you are looking for within linux (perhaps a bash script, instead?): $ hamachi list | grep -oP '25\.\d+\.\d+\.\d+' 25.0.0.0 25.255.255.255 For your python script, you want to group your regex: reg = re.compile(r'(25\.\d+\.\d+\.\d+)', re.MULTILINE) So when you call group(1) or group(0), it'll grab just the addresses. On Thu, Oct 2, 2014 at 12:33 PM, David Rock wrote: > * Bo Morris [2014-10-02 11:41]: >> Hello all, hope everyone is doing well. >> >> When I run the linux command "hamachi list" i get something along the lines >> of the following output >> >>087-888-279 Pandora25.x.x.xxx alias: not >> set >>096-779-867 AM1LaptopBD-PC25.x.x.xxx alias: not set >>097-552-220 OWS-Desktop 125.0.0.0 alias: not set >>099-213-641 DESKTOP 25.0.0.0 alias: not set >> >> I am trying to write a python script that will run the above command and >> only print out the IP's that begin with 25. How do I strip out all other >> text except for the IP's that begin with "25?" > > There are a few assumptions that need to be made, for starters. > > Is the format always the same (ie, is the IP address always in column 3 > separated by whitespace)? Looking at the line with "OWS-Desktop 1", the > answer is no. That complicates things a bit. If it was true, you could > use the string split method to get column 3. Maybe the fields are > separated by a tab? > > A regex may be possible, but you will have similar issues to using > split. > > I'm also assuming it's possible for there to be IP addresses that do not > start with 25. Are you looking to isolate those? > > It's not necessary to write out to a file first. You can get the output > from commands and work on it directly. > > Another approach would be to change the command you are running. I've > never heard of hamachi list before; does it have any commandline options > to display only IP addresses? > > -- > David Rock > da...@graniteweb.com > ___ > Tutor maillist - Tutor@python.org > To unsubscribe or change subscription options: > https://mail.python.org/mailman/listinfo/tutor ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Iterating Lines in File and Export Results
Alan, Peter, et al:
Thank you all very much! Staring at this problem for hours was driving
me crazy and I am very appreciative for your guys' time in looking
into my silly error -- I have thoroughly reviewed both the responses
and it makes perfect sense (*sigh of relief*).
On Thu, Oct 2, 2014 at 6:08 PM, Peter Otten <__pete...@web.de> wrote:
> John Doe wrote:
>
>> Hello List,
>> I am in need of your assistance. I have a text file with random words
>> in it. I want to write all the lines to a new file. Additionally, I am
>> using Python 2.7 on Ubuntu 12.04:
>>
>> Here is my code:
>>
>> def loop_extract():
>> with open('words.txt', 'r') as f:
>> for lines in f:
>
> The name `lines` is misleading, you are reading one line at a time.
>
>> #print lines (I confirmed that each line is successfully
>> #printed)
>> with open('export.txt', 'w') as outf:
>> outf.write(lines)
>> #outf.write(lines)
>> #outf.write('{}\n'.format(lines))
>> #outf.write('{}\n'.format(line for line in lines))
>>
>>
>> For some reason, the second file only contains the last line from the
>> original file -- I have tried multiple variations (.read, .readlines,
>> .writelines, other examples preceded by comment from above and many
>> more) and tried to use the module, fileinput, but I still get the same
>> results.
>
> Every time the line
>
>> with open('export.txt', 'w') as outf:
>
> is executed the file "export.txt" is truncated:
>
> https://docs.python.org/dev/library/functions.html#open
>
> To avoid the loss of data open the file once, outside the loop:
>
> with open("words.txt") as infile, open("export.txt", "w") as outfile:
> for line in infile:
> outfile.write(line)
>
>
>> I do understand there is another way to copy the file over, but to
>> provide additional background information on my purpose -- I want to
>> read a file and save successful regex matches to a file; exporting
>> specific data. There doesn't appear to be anything wrong with my
>> expression as it prints the expected results without failure. I then
>> decided to just write the export function by itself in its basic form,
>> per the code above, which the same behavior occurred;
>
> That is a good approach! Reduce the code until only the source of the
> problem is left.
>
>> only copying the
>> last line. I've googled for hours and, unfortunately, at loss.
>
> I do that too, but not "for hours" ;)
>
>> I want to read a file and save successful regex matches to a file;
>> exporting specific data.
>
> An experienced user of Python might approach this scenario with a generator:
>
> def process_lines(infile):
> for line in infile:
> line = process(line) # your line processing
> if meets_condition(line): # your filter condition
> yield line
>
> with open("words.txt") as infile:
> with open("export.txt", "w") as outfile:
> outfile.writelines(
> process_lines(infile))
>
>
> ___
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