Re: [Tutor] decorators -- treat me like i'm 6.. what are they.. why are they?

[Steven D'Aprano]

On Wed, Jul 06, 2016 at 03:35:16PM -0400, bruce wrote:
> Hi.
> 
> Saw the decorator thread earlier.. didn't want to pollute it. I know, 
> I could google!
> 
> But, what are decorators, why are decorators? who decided you needed 
> them!

Sometimes you find yourself writing many functions (or methods) that 
have bits in common.

For example, you might have a bunch of mathematical functions that all 
start off with the same boring code "check that the argument x is a 
number, raise an exception if it isn't". Sure, it's only two or three 
lines, but if you've got twenty functions to write, you have to write 
those same lines twenty times. And then if you decide to change the 
error message, or change the way you check something is a number, you 
have to do that in twenty places.

A decorator let's you factor out that common code into a single place, 
so you only need to make changes to *one* place instead of twenty 
separate functions. You still have to apply the decorator to each of the 
functions, but that's only once line. Besides, a decorator is not really 
about cutting back the number of lines of code (although it is nice when 
that happens) as it is about moving common code to one place.

def verify_numeric_argument(func):
"""Decorator that checks the argument to a function is a number."""
@functools.wraps(func)
def inner(x):
if isinstance(x, numbers.Number):
return func(x)
else:
kind = type(x).__name__
raise TypeError("expected a number, but got %s" % kind)
return inner


@verify_numeric_argument
def sinc(x):
if x == 0:
return 1.0
else:
return math.sin(x)/x



Obviously you wouldn't bother if you're only going to do it *once*. 
There's a bunch of overhead involved in writing a decorator, usually 
about five or six lines of code per decorator. The rule of thumb I use 
is that if I have one or two functions with the same, common, chunk of 
code, I probably wouldn't bother; if I have three functions, I might, 
and if I have four or more, then I probably will.

But as I said, it's not really about saving lines of code. It's about 
keeping code that belongs together in one place, and about recognising 
when functions repeat the same structure. If you have a bunch of 
functions that look like this:

def function(arguments):
boring boilerplate that is the same each time
interesting bit that is different each time
more boring boilingplate
return result

then this is a great candidate for a decorator: move the boring 
boilerplate into the decorator, then turn the functions into this:

@apply_boilerplate
def function(arguments):
interesting bit that is different each time
return result


Doesn't that look better? That's the purpose of the decorator: move the 
boring bits, or sometimes the hard, complicated bits, away so you can 
focus on the interesting bits. Depending on just how boring that 
boilerplate is, I might even do it for ONE function, just to get it out 
of the way.


-- 
Steve
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Re: [Tutor] error

[Danny Yoo]

>
> Have you checked that you have the requisite permissions? That the
> socket you are connecting to exists? If its a system call error the
> problem is most likely in your environment rather than your code.
>


That particular error is from Windows.  One common cause for it is a
network firewall, which normally is great because at least it's
working as a first line of defense.  If that's the case, you probably
want to tell your firewall to make an exception for your program.
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Re: [Tutor] decorators -- treat me like i'm 6.. what are they.. why are they?

[Danny Yoo]

> As to who suggested them you'd need to go back through the
> PEPs to see who first suggested it, and then maybe more to see
> who's idea finally got accepted. I think it was in Python 2.5.


Hi Bruce,


Yes, it happened back around 2003:

https://www.python.org/dev/peps/pep-0318/

Decorators are "syntactic sugar" in the sense that they can be
rewritten in terms of more primitive parts of the language (function
calls).  If you already know about functions that work on functions,
then you know what decorators are.

However, if you haven't seen the concept of functions that take
functions as input, and you want to talk about that, feel free to ask
questions!  It would likely be a "fun" discussion to have.
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Re: [Tutor] OS and Windows version

[Alan Gauld via Tutor]

On 06/07/16 19:34, Moses, Samuel wrote:
> Mine
> 
> OS: Windows
> Windows version: 8.1
> 
> Python 3.2
> Wing IDE: 15.1

Thanks for the extra info but it doesn't help much with
your problem since we still don't know what your code
does nor how your environment is set up.

BTW Can you connect to your target socket using telnet? (Use
a CMD console)If you get an error message there you probably
can't connect from Python either.

Finally you could try running the script outside Wing. IDEs can
do all sorts of weird stuff to programs. Its always worth a try
running it using the vanilla Python interpreter.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] decorators -- treat me like i'm 6.. what are they.. why are they?

[Alan Gauld via Tutor]

On 06/07/16 20:35, bruce wrote:

> Saw the decorator thread earlier.. didn't want to pollute it. I know, I
> could google!
> 
> But, what are decorators, why are decorators? who decided you needed them!

decorators are things that modify functions in standard ways.
Specifically they are functions that act on functions.
Mostly people think of them as being the thing that comes after an @
sign, but in fact they are just functions but with a bit of syntactic
sugar to make them more readable.

You don't really *need* them as a feature, but if you have
the ability to treat functions as values they are a useful
technique.

Most Python programmers don't implement decorators they just
use the standard ones. But as the thread showed implementing
them is not too difficult once you get happy with the idea
of passing functions around as arguments to other functions.

As to who suggested them you'd need to go back through the
PEPs to see who first suggested it, and then maybe more to see
who's idea finally got accepted. I think it was in Python 2.5.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] decorators -- treat me like i'm 6.. what are they.. why are they?

[Alex Hall]

On Wed, Jul 6, 2016 at 4:56 PM, Alex Hall  wrote:

>
>
> On Wed, Jul 6, 2016 at 3:35 PM, bruce  wrote:
>
>> Hi.
>>
>> Saw the decorator thread earlier.. didn't want to pollute it. I know, I
>> could google!
>>
>> But, what are decorators, why are decorators? who decided you needed them!
>>
>
> I thought of an example that may help. It's not a great example, but here
goes.

Say you own a bakery, and you employ Joe and me. You task me with taking
cookies from the kitchen and putting them in the display case in the
service area, which I do well. The problem is that I don't know what looks
appealing and what doesn't, so I keep putting cookies out that don't look
great. You can't teach something like that, it's just something you know.

Since you can't modify me to do my task in the way you want, you grab Joe
and have him help me. Instead of putting all the cookies out, I now have
Joe checking me before I place each one. Joe knows appealing, so is good at
this, but he's also usually busy up front so can't carry the cookies out
from the kitchen like I can.

Joe is the decorator, and I'm the function being decorated. Since I do my
task well, but lack a component you want, and since that component would be
hard to program, you find an existing version of the component (Joe) and
tell us to work together. Joe can modify my output before I return it,
letting him filter the appealing cookies out while I do the work of
carrying them up and setting out the ones Joe doesn't tell me to reject.
Hopefully this makes some sense.

> They're functions that modify the decorated function. If I make a function
> that performs a task, I might decorate with a logging function:
>
> @logThis
> def doSomething():
>   #do stuff
>
> The logThis decorator could log the run, or modify the doSomething output,
> or do any number of tasks. To get that extra functionality, I need only
> decorate my own function. I see these a lot in Flask, a web framework,
> where they are used to define what parts of your website are handled by
> what functions, for instance. Basically, they let you extend what your
> functions do without needing to subclass anything.
>
>>
>> Thanks!
>> ___
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>>
>
>
>
> --
> Alex Hall
> Automatic Distributors, IT department
> ah...@autodist.com
>



-- 
Alex Hall
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ah...@autodist.com
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[Tutor] OS and Windows version

[Moses, Samuel]

Mine

OS: Windows

Windows version: 8.1

Python 3.2

Wing IDE: 15.1

Sam
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Re: [Tutor] decorators -- treat me like i'm 6.. what are they.. why are they?

[Alex Hall]

On Wed, Jul 6, 2016 at 3:35 PM, bruce  wrote:

> Hi.
>
> Saw the decorator thread earlier.. didn't want to pollute it. I know, I
> could google!
>
> But, what are decorators, why are decorators? who decided you needed them!
>

They're functions that modify the decorated function. If I make a function
that performs a task, I might decorate with a logging function:

@logThis
def doSomething():
  #do stuff

The logThis decorator could log the run, or modify the doSomething output,
or do any number of tasks. To get that extra functionality, I need only
decorate my own function. I see these a lot in Flask, a web framework,
where they are used to define what parts of your website are handled by
what functions, for instance. Basically, they let you extend what your
functions do without needing to subclass anything.

>
> Thanks!
> ___
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> To unsubscribe or change subscription options:
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>



-- 
Alex Hall
Automatic Distributors, IT department
ah...@autodist.com
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[Tutor] decorators -- treat me like i'm 6.. what are they.. why are they?

[bruce]

Hi.

Saw the decorator thread earlier.. didn't want to pollute it. I know, I
could google!

But, what are decorators, why are decorators? who decided you needed them!

Thanks!
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Re: [Tutor] error

[Alan Gauld via Tutor]

On 06/07/16 18:27, Moses, Samuel wrote:
> I am getting an error.  I tired to run the script in wing IDE.
> 

Without the accompanying code we can only guess.

> I am getting this error,
> 
> "Traceback (most recent call last):
>   File "C:\Program Files (x86)\Wing IDE 5.1\bin\wingdb.py", line 822, in
> main
> args['firststop'], err, netserver, pwfile_path)
>   File "C:\Program Files (x86)\Wing IDE 5.1\bin\wingdb.py", line 649, in
> CreateServer
> pwfile_path, internal_modules=tuple(internal_modules))
>   File "C:\src\ide\bin\3.2\src/debug/tserver\netserver.pyc", line 922, in
> __init__
>   File "C:\src\ide\bin\3.2\src/debug/tserver\netserver.pyc", line 2234, in
> __PrepareConnectListener
>   File "C:\Python32\lib\socket.py", line 94, in __init__
> _socket.socket.__init__(self, family, type, proto, fileno)
> socket.error: [Errno 10107] A system call has failed"

Have you checked that you have the requisite permissions? That the
socket you are connecting to exists? If its a system call error the
problem is most likely in your environment rather than your code.

But check the values you pass into socket. Print them out
to check they are what you (and Python) expect...

> I tried printing "Christmas"

I have no idea what that means. Literally putting

print "Christmas"

in your code probably wouldn't help so I assume you
did something else?

> It said there was no TCP/IP connection.

What said? Was it another error trace? Or something else?

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] help with exercise 15 of zed shaw's LPTHW

[Alan Gauld via Tutor]

On 06/07/16 15:04, loh...@tuta.io wrote:

> script, filename = argv
> txt = open (filename)
> 
> print "Here's your file %r: " % filename
> print txt.read()
> 
> print "Type the filename again: "
> file_again = raw_input("> ")
> 
> txt_again = open(file_again)
> print txt_again.read()


> why do I have to create a variable txt_again to assign it to the open 
> function and them print the file?

You don't, and could get away with a single
variable - filename. Like this:

filename = argv[1]
print "Here's your file %r: " % filename
print open(filename).read()

filename = raw_input("Type the filename again: >")
print open(filename).read()

But your book is (I assume) trying to teach you
good practice.

While you could have just printed the result of read directly,
its better not to over complicate code by doing too much in one
line (for a start, its harder to debug) the same variable.
Variables are cheap to create and if given useful names
tell us a lot about the purpose of the code.

In this case it's all a bit trivial, just printing the file
content, but if you were doing more complex processing
storing the file (and data) in variables would be the
best choice.

-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos


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Re: [Tutor] Subclassing logging.Handler?

[Alex Hall]

On Wed, Jul 6, 2016 at 1:01 PM, Alex Hall  wrote:

> Regarding this project: I've gone ahead and tried a variant of it. I
> wanted to log to an HTML file, since those are much easier to look at with
> a screen reader and so I could get used to the concepts involved. Here's
> what I've come up with so far. I'll ask the question, then paste the code.
> I'm getting an error on the self.stream.write line in _open that
> "'ADTimedRotatingLogFileHandler' has no attribute stream". If I'm
> subclassing TimedRotatingFileHandler, how can it not have stream?
>

Again, in case someone is searching for an answer later and finds this
thread: you must assign it.

def _open(self):
self.stream = super(ADTimedRotatingLogFileHandler, self)._open()

I'm now getting a different error, in close():
TypeError: must be type, not None

I'm not sure what that's about, but at least I now know why I wasn't
getting a stream.

>
> import logging
> import logging.handlers as logHandlers
> class ADTimedRotatingLogFileHandler(logHandlers.TimedRotatingFileHandler):
>
> def __init__(self, filename, when, interval, backupCount, title):
> """Most parameters are for the superclass, but 'title' is the
> title you want your HTML file to have."""
> super(ADTimedRotatingLogFileHandler, self).__init__(filename,
> when, interval, backupCount)
> self._title = title
>
> def _open(self):
> super(ADTimedRotatingLogFileHandler, self)._open()
> self.stream.write("""
> 
> %s
> 
> 
> """ %(self._title))
>
> def close(self):
> self.stream.write("""
> 
> """)
> super(ADTimedRotatingLogFileHandler, self).close()
>
>
> On Wed, Jul 6, 2016 at 8:32 AM, Alex Hall  wrote:
>
>> Hey list,
>> Another day, another Python experiment. I'm wondering what methods I'd
>> have to implement in a custom subclass of logger.Handler.
>>
>> Currently, the recurring jobs I have written log their events to a file
>> each time they run. That's fine, but it doesn't let me keep
>> easily-sorted/searched records. Even if I use a size-based log file
>> handler, it'll get hard to search. I'm pondering logging to a database as
>> well, so that the files will always have the most recent few runs, but the
>> database will have everything. That means subclassing logger.Handler.
>>
>> I found the docs for this, but is emit() the only function I need to
>> implement? There are things about i/o locks, formatting, and so on as well.
>> How much do I need to do, and how much can I leave up to the super class?
>> I'll have to call
>> super(MyLogHandler, self).__init__()
>> I know, but what else do I have to worry about? To be clear, I'm not
>> asking about logging to a database, only what to do to make a Handler
>> subclass capable of logging through whatever mechanisms I want. Thanks.
>>
>> --
>> Alex Hall
>> Automatic Distributors, IT department
>> ah...@autodist.com
>>
>
>
>
> --
> Alex Hall
> Automatic Distributors, IT department
> ah...@autodist.com
>



-- 
Alex Hall
Automatic Distributors, IT department
ah...@autodist.com
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[Tutor] error

[Moses, Samuel]

I am getting an error.  I tired to run the script in wing IDE.

I am getting this error,

"Traceback (most recent call last):
  File "C:\Program Files (x86)\Wing IDE 5.1\bin\wingdb.py", line 822, in
main
args['firststop'], err, netserver, pwfile_path)
  File "C:\Program Files (x86)\Wing IDE 5.1\bin\wingdb.py", line 649, in
CreateServer
pwfile_path, internal_modules=tuple(internal_modules))
  File "C:\src\ide\bin\3.2\src/debug/tserver\netserver.pyc", line 922, in
__init__
  File "C:\src\ide\bin\3.2\src/debug/tserver\netserver.pyc", line 2234, in
__PrepareConnectListener
  File "C:\Python32\lib\socket.py", line 94, in __init__
_socket.socket.__init__(self, family, type, proto, fileno)
socket.error: [Errno 10107] A system call has failed"

I tried printing "Christmas"
It said there was no TCP/IP connection.

Any suggestion would help

Thank you

Samuel Moses
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Re: [Tutor] help with exercise 15 of zed shaw's LPTHW

[Michael Selik]

On Wed, Jul 6, 2016 at 10:59 AM  wrote:

> why do I have to create a variable txt_again to assign it to the open
> function and them print the file?
> why is it that I can't only write something like open(file_again).read()?
>

Good insight. In fact you don't need to create the variable. The code ``data
= open('filename').read()`` will open the file named "filename" in the
current working directory, read it, and assign the data to a variable.

However, many programmers use variables not because they must, but because
good variable names can make code easier to read. Also, doing less stuff on
a line of code can make that code easier to read.
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Re: [Tutor] Subclassing logging.Handler?

[Alex Hall]

Regarding this project: I've gone ahead and tried a variant of it. I wanted
to log to an HTML file, since those are much easier to look at with a
screen reader and so I could get used to the concepts involved. Here's what
I've come up with so far. I'll ask the question, then paste the code. I'm
getting an error on the self.stream.write line in _open that
"'ADTimedRotatingLogFileHandler' has no attribute stream". If I'm
subclassing TimedRotatingFileHandler, how can it not have stream?

import logging
import logging.handlers as logHandlers
class ADTimedRotatingLogFileHandler(logHandlers.TimedRotatingFileHandler):

def __init__(self, filename, when, interval, backupCount, title):
"""Most parameters are for the superclass, but 'title' is the title
you want your HTML file to have."""
super(ADTimedRotatingLogFileHandler, self).__init__(filename, when,
interval, backupCount)
self._title = title

def _open(self):
super(ADTimedRotatingLogFileHandler, self)._open()
self.stream.write("""

%s


""" %(self._title))

def close(self):
self.stream.write("""

""")
super(ADTimedRotatingLogFileHandler, self).close()


On Wed, Jul 6, 2016 at 8:32 AM, Alex Hall  wrote:

> Hey list,
> Another day, another Python experiment. I'm wondering what methods I'd
> have to implement in a custom subclass of logger.Handler.
>
> Currently, the recurring jobs I have written log their events to a file
> each time they run. That's fine, but it doesn't let me keep
> easily-sorted/searched records. Even if I use a size-based log file
> handler, it'll get hard to search. I'm pondering logging to a database as
> well, so that the files will always have the most recent few runs, but the
> database will have everything. That means subclassing logger.Handler.
>
> I found the docs for this, but is emit() the only function I need to
> implement? There are things about i/o locks, formatting, and so on as well.
> How much do I need to do, and how much can I leave up to the super class?
> I'll have to call
> super(MyLogHandler, self).__init__()
> I know, but what else do I have to worry about? To be clear, I'm not
> asking about logging to a database, only what to do to make a Handler
> subclass capable of logging through whatever mechanisms I want. Thanks.
>
> --
> Alex Hall
> Automatic Distributors, IT department
> ah...@autodist.com
>



-- 
Alex Hall
Automatic Distributors, IT department
ah...@autodist.com
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[Tutor] Subclassing logging.Handler?

[Alex Hall]

Hey list,
Another day, another Python experiment. I'm wondering what methods I'd have
to implement in a custom subclass of logger.Handler.

Currently, the recurring jobs I have written log their events to a file
each time they run. That's fine, but it doesn't let me keep
easily-sorted/searched records. Even if I use a size-based log file
handler, it'll get hard to search. I'm pondering logging to a database as
well, so that the files will always have the most recent few runs, but the
database will have everything. That means subclassing logger.Handler.

I found the docs for this, but is emit() the only function I need to
implement? There are things about i/o locks, formatting, and so on as well.
How much do I need to do, and how much can I leave up to the super class?
I'll have to call
super(MyLogHandler, self).__init__()
I know, but what else do I have to worry about? To be clear, I'm not asking
about logging to a database, only what to do to make a Handler subclass
capable of logging through whatever mechanisms I want. Thanks.

-- 
Alex Hall
Automatic Distributors, IT department
ah...@autodist.com
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Re: [Tutor] help with exercise 15 of zed shaw's LPTHW

[lohecn]

first, sorry everyone for having attached the file instead of just typing it 
here.
second, thanks a lot for the replies; even though I gave you no code it was 
quite helpful!
the code was this:

from sys import argv

script, filename = argv
txt = open (filename)

print "Here's your file %r: " % filename
print txt.read()

print "Type the filename again: "
file_again = raw_input("> ")

txt_again = open(file_again)

print txt_again.read()

Peter Otten explained it to me line by line [thanks so much :)] however, I do 
have one more question:
why do I have to create a variable txt_again to assign it to the open 
function and them print the file?
why is it that I can't only write something like open(file_again).read()?


6. Jul 2016 05:22 by __pete...@web.de:


> loh...@tuta.io>  wrote:
>
>> hey everyone. this is my first time trying this -- actually, I've been
>> studying python only for some days now, and I'm afraid my questions are
>> going to be rally simple, but I can't seem to understand this piece of
>> code and thus can't move on.
>
> You seem to be talking about
>
> http://learnpythonthehardway.org/book/ex15.html
>
> """
> from sys import argv
>
> script, filename = argv
>
> txt = open(filename)
>
> print "Here's your file %r:" % filename
> print txt.read()
>
> print "Type the filename again:"
> file_again = raw_input("> ")
>
> txt_again = open(file_again)
>
> print txt_again.read()
> """
>
> As others said, always provide the code you are asking about, or if that is
> not possible at least provide a link.
>
>> you probably know the book, so you know that zed always makes us write
>> code so that then we can understand how it works, and it's great, but in
>> this exercise there are just too many new functions and without
>> explanation they are a bit hard to understand... so I'm having trouble
>> with most of the lines here.
>>
>> it's not that I want the full explanation to that code, but since I'm
>> unfamiliar with some of its concepts, I'm just going to tell you all the
>> things that I don't understand (sorry for it being a lot):
>> 1. the need to put script into an estipulation for argv (line 3)
>
> Write a script tmp.py containing
>
> from sys import argv
> print argv
>
> then call it with with one parameter, e. g.
>
> $ python tmp.py somefile.txt
> ['tmp.py', 'somefile.txt']
>
> As you can see argv is a list with two items, the first being "tmp.py", the
> name of the script you are invoking. You are only interested in the second
> one, the filename. The easy way to get that is
>
> filename = argv[1]
>
> the hard way is to use "unpacking"
>
> script, filename = argv
>
> where python will assign one item from the list on the right to every name
> on the left:
>
 items = ["foo", "bar"]
 one, two = items
 one
> 'foo'
 two
> 'bar'
>
> What happens if the number of names on the left doesn't match the number of
> items in the list?
>
 one, two, three = items
> Traceback (most recent call last):
>   File "", line 1, in 
> ValueError: need more than 2 values to unpack
>
> You get an exception. That is why you have to provide the name "script" in
> Zed's example even though you are not actually interested in the script
> name.
>
>> 2. the what is txt and why it has to be used there (line 4)
>
> txt is a file object and
>
>> 3. txt.read() -- which are all new functions(? I dont even know what they
>> are)  (line 7)
>
> read() is a method that here reads the whole file into a string. You use 
> the
> open() function to open a file and usually assign the file object that is
> returned by open to a name. You can freely choose that name. The structure
> is the same for every object, be it a number:
>
> x = 42  # assign a number to x
> y = x + x  # do some arithmetic with x and assign the result to y
> print y  # print the result
>
> a list:
>
> mynumbers = list()  # create a list
> mynumbers.append(42)  # append a number to the list
> print mynumbers  # print the list
>
> or a file:
>
> myfile = open("example.txt")  # open the file example.txt in the current
>   # working directory. If the file doesn't 
> exist
>   # you get an error
>
> print "first line:", myfile.readline()  # read the first line and print it
> print "rest of the file:"
> print myfile.read()  # read the rest of the file and print it
>
> myfile.close()  # close the file
>
>> 4. file_again (line 10)
>> 5. txt_again (line 12)
>> and line 14.
>
> 4. and 5. are just a repetition of the first part, with the variation that
> the filename, assigned to file_again is read interactively with raw_input()
> instead of passing it as a commandline argument to the script.
>
> The names used can be freely chosen by the programmer, a script
>
> from sys import argv
>
> red_apple, my_hat = argv
>
> blue_suede_shoes = open(my_hat)
> print blue_suede_shoes.read()
> blue_suede_shoes.close()
>
> would work exactly like the first part of the hard-way example. However,
> picking 

Re: [Tutor] help with exercise 15 of zed shaw's LPTHW

[Peter Otten]

loh...@tuta.io wrote:

> hey everyone. this is my first time trying this -- actually, I've been
> studying python only for some days now, and I'm afraid my questions are
> going to be rally simple, but I can't seem to understand this piece of
> code and thus can't move on.

You seem to be talking about

http://learnpythonthehardway.org/book/ex15.html

"""
from sys import argv

script, filename = argv

txt = open(filename)

print "Here's your file %r:" % filename
print txt.read()

print "Type the filename again:"
file_again = raw_input("> ")

txt_again = open(file_again)

print txt_again.read()
"""

As others said, always provide the code you are asking about, or if that is 
not possible at least provide a link.
 
> you probably know the book, so you know that zed always makes us write
> code so that then we can understand how it works, and it's great, but in
> this exercise there are just too many new functions and without
> explanation they are a bit hard to understand... so I'm having trouble
> with most of the lines here.
> 
> it's not that I want the full explanation to that code, but since I'm
> unfamiliar with some of its concepts, I'm just going to tell you all the
> things that I don't understand (sorry for it being a lot):
> 1. the need to put script into an estipulation for argv (line 3)

Write a script tmp.py containing

from sys import argv
print argv

then call it with with one parameter, e. g.

$ python tmp.py somefile.txt
['tmp.py', 'somefile.txt']

As you can see argv is a list with two items, the first being "tmp.py", the 
name of the script you are invoking. You are only interested in the second 
one, the filename. The easy way to get that is

filename = argv[1]

the hard way is to use "unpacking"

script, filename = argv

where python will assign one item from the list on the right to every name 
on the left:

>>> items = ["foo", "bar"]
>>> one, two = items
>>> one
'foo'
>>> two
'bar'

What happens if the number of names on the left doesn't match the number of 
items in the list?

>>> one, two, three = items
Traceback (most recent call last):
  File "", line 1, in 
ValueError: need more than 2 values to unpack

You get an exception. That is why you have to provide the name "script" in 
Zed's example even though you are not actually interested in the script 
name.

> 2. the what is txt and why it has to be used there (line 4)

txt is a file object and

> 3. txt.read() -- which are all new functions(? I dont even know what they
> are)  (line 7)

read() is a method that here reads the whole file into a string. You use the 
open() function to open a file and usually assign the file object that is 
returned by open to a name. You can freely choose that name. The structure 
is the same for every object, be it a number:

x = 42  # assign a number to x
y = x + x  # do some arithmetic with x and assign the result to y
print y  # print the result

a list:

mynumbers = list()  # create a list
mynumbers.append(42)  # append a number to the list
print mynumbers  # print the list

or a file:

myfile = open("example.txt")  # open the file example.txt in the current
  # working directory. If the file doesn't exist
  # you get an error

print "first line:", myfile.readline()  # read the first line and print it
print "rest of the file:"
print myfile.read()  # read the rest of the file and print it

myfile.close()  # close the file

> 4. file_again (line 10)
> 5. txt_again (line 12)
> and line 14.

4. and 5. are just a repetition of the first part, with the variation that 
the filename, assigned to file_again is read interactively with raw_input() 
instead of passing it as a commandline argument to the script.

The names used can be freely chosen by the programmer, a script

from sys import argv

red_apple, my_hat = argv

blue_suede_shoes = open(my_hat)
print blue_suede_shoes.read()
blue_suede_shoes.close()

would work exactly like the first part of the hard-way example. However, 
picking descriptive names and using them consistently makes it much easier 
for a human reader to understand what's going on.

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Re: [Tutor] help with exercise 15 of zed shaw's LPTHW

[Alan Gauld via Tutor]

On 06/07/16 00:56, loh...@tuta.io wrote:
> hey everyone. this is my first time trying this 

Welcome, but...

> you probably know the book,

Sorry, I don't and I suspect I'm not alone.
It's probably a fine book, but we don't all know it.

>  so you know that zed always makes us write code 
> so that then we can understand how it works, 

Good man Zed :-)

> exercise there are just too many new functions and without explanation they 
> are a bit hard to understand... so I'm having trouble with most of the lines 
> here.

And here's the next problem.
This is a text based mailing list. As such the server often strips out
attachments as potential security risks. So we can't see the code (I'm
assuming you attached it?)

> 1. the need to put script into an estipulation for argv (line 3)
> 2. the what is txt and why it has to be used there (line 4)
> 3. txt.read() -- which are all new functions(? I dont even know what they 
> are)  (line 7)
> 4. file_again (line 10)
> 5. txt_again (line 12)
> and line 14.

Without sight of the code its hard to know what's going on.
But I suspect some of these "functions" are actually variables
(or objects) and hopefully zed has already discussed those?

Can you repost but include your code inside the mail message.
Also try to post in plain text since HTML tends to get garbled
in transit.

HTH
-- 
Alan G
Author of the Learn to Program web site
http://www.alan-g.me.uk/
http://www.amazon.com/author/alan_gauld
Follow my photo-blog on Flickr at:
http://www.flickr.com/photos/alangauldphotos

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Re: [Tutor] dont understand part of a code

[Michael Selik]

On Tue, Jul 5, 2016 at 5:36 PM Michael Selik 
wrote:

> On Sat, Jul 2, 2016 at 8:29 AM Alan Gauld via Tutor 
> wrote:
>
>> There are arguably easier ways of doing this
>>
>
> I think you'll find that for-loops are preferable to while-loops. Here's
> an alternative implementation.
>
> https://gist.github.com/selik/d8e0a7622ceff0fe8984a7d19d44bfca
>

On further reflection, unfortunately a for-loop doesn't seem best for this
particular problem. I updated the gist, linked above. I wish the author had
chosen a better problem to emphasize Pythonic iteration.
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Re: [Tutor] help with exercise 15 of zed shaw's LPTHW

[Michael Selik]

On Tue, Jul 5, 2016 at 8:24 PM  wrote:

> I'm having trouble with most of the lines here.
>

It looks like you tried to attach a file. This mailing list does not allow
attachments. Instead, could you paste the code into your email?


> things that I don't understand:
> 1. the need to put script into an estipulation for argv (line 3)
> 2. the what is txt and why it has to be used there (line 4)
> 3. txt.read() -- which are all new functions(? I dont even know what they
> are)  (line 7)
>

I'm guessing txt is a file object or a file-like object that supports the
.read method to read the entire contents of the file into a single string
object.


> 4. file_again (line 10)
> 5. txt_again (line 12)
> and line 14.
>
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Re: [Tutor] isinstance versus 'is'?

[Alex Hall]

Thanks everyone, that all makes more sense. I think I was indeed thinking of 
"is None", which is essentially the same as "== None" (I know there's a subtile 
difference, but they do the same thing). Of course, "is None" fits with this 
usage, as you're asking if the value is the literal None object. It seems it's 
the way None is handled, not an exception in the way 'is' works. Anyway, thanks 
for the explanations.
> On Jul 5, 2016, at 20:54, Steven D'Aprano  wrote:
> 
> On Tue, Jul 05, 2016 at 03:05:45PM -0400, Alex Hall wrote:
> 
> a = 5
> isinstance(a, int)
>> True
> a is int
>> False
>> 
>> What happened there? Don't these do the same thing? I thought I could use
>> them interchangeably?
> 
> You're probably thinking of "is a", as in, "5 is an int", "'Hello 
> World' is a str", "[1, 2, 3] is a list", etc.
> 
> Python doesn't have an operator for testing "is a" relationships, it 
> uses isinstance(obj, type). There's also issubclass(), for testing 
> whether one class is a subclass of another.
> 
> "x is y" checks whether the two operands x and y are the same object. 
> That's *not* the same as checking whether they are equal. You should 
> hardly ever use "is" in Python, with the exception of testing for None: 
> "if obj is None: ..." sort of thing.
> 
> 
> -- 
> Steve
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