Re: x, = y (???)
kj wrote:
I just came across an assignment of the form
x, = y
where y is a string (in case it matters).
1. What's the meaning of the comma in the LHS of the assignment?
2. How could I have found this out on my own?
(Regarding (2) above, I consulted the index of several Python
reference books but I could not find the answer to (1). I hope to
find a better Python reference!)
TIA!
kynn
Try this:
y = 'abc'
type(y)
type 'str'
type((y,))
type 'tuple'
y = y, # y, is just short for (y,) you *have* to use a coma in 1
length tuples
y
('abc',)
type(y)
type 'tuple'
y[0]
'abc'
x, = y #same as x = y[0] OR more verbosely (x,) = (y,)
x
'abc'
type(x)
type 'str'
Maybe that will hape you understand what the comma is for?
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Re: x, = y (???)
In [EMAIL PROTECTED] Matthew Woodcraft [EMAIL PROTECTED] writes: kj wrote: I still don't get it. If we write y = 'Y' x, = y what's the difference now between x and y? And if there's no difference, what's the point of performing such unpacking? If y really is is a string, I think it's likely that the line you came across was a typo. OK, this is the best explanation I've seen for the code I'm talking about. This code may be found at: http://norvig.com/sudo.py in the definition of the function eliminate. Here's the fragment: elif len(values[s]) == 1: ## If there is only one value (d2) left in square, remove it from peers d2, = values[s] Now, in the assignment, values[s] *is* a string of length 1. So this assignment appears to me entirely equivalent (in its ultimate effect) to the more straightforward d2 = values[s] ...but, since I'm a noob, I thought I'd ask :-) Thanks! kynn -- NOTE: In my address everything before the first period is backwards; and the last period, and everything after it, should be discarded. -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
kj wrote: I just came across an assignment of the form x, = y where y is a string (in case it matters). 1. What's the meaning of the comma in the LHS of the assignment? 2. How could I have found this out on my own? (Regarding (2) above, I consulted the index of several Python reference books but I could not find the answer to (1). I hope to find a better Python reference!) It's unpacking a 1-tuple: (x,) = y The parentheses here are not necessary and are sometimes left out. -- Erik Max Francis [EMAIL PROTECTED] http://www.alcyone.com/max/ San Jose, CA, USA 37 18 N 121 57 W AIM, Y!M erikmaxfrancis The United States, while they wish for war with no nation, will buy peace with none. -- James Madison -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
On Thu, Jul 17, 2008 at 2:55 PM, kj [EMAIL PROTECTED] wrote: In [EMAIL PROTECTED] Erik Max Francis [EMAIL PROTECTED] writes: kj wrote: I just came across an assignment of the form x, = y where y is a string (in case it matters). 1. What's the meaning of the comma in the LHS of the assignment? It's unpacking a 1-tuple: (x,) = y The parentheses here are not necessary and are sometimes left out. I still don't get it. If we write y = 'Y' x, = y what's the difference now between x and y? And if there's no difference, what's the point of performing such unpacking? TIA! y = (1,) y (1,) x, = y x 1 -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
On Jul 17, 12:55 pm, kj [EMAIL PROTECTED] wrote:
I still don't get it. If we write
y = 'Y'
x, = y
what's the difference now between x and y? And if there's no
difference, what's the point of performing such unpacking?
Try:
y = abc
x, = y
You were unpacking y into ('Y',)
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Re: x, = y (???)
On Jul 17, 12:55 pm, kj [EMAIL PROTECTED] wrote: what's the point of performing such unpacking? record = [name, address, telephone] ... name, address, telephone = record -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
kj wrote: I still don't get it. If we write y = 'Y' x, = y what's the difference now between x and y? And if there's no difference, what's the point of performing such unpacking? If y really is is a string, I think it's likely that the line you came across was a typo. In the case you give above, there's no difference at the end between x and y. If y had length other than 1, the second line would raise an exception, so it's not the same as plain x = y. But if that's the intended effect, it's a daft way to write it. -M- -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
kj [EMAIL PROTECTED] wrote: I still don't get it. If we write y = 'Y' x, = y what's the difference now between x and y? And if there's no difference, what's the point of performing such unpacking? None whatsoever when the string has only one character, but with 2 characters it becomes an obfuscated way of checking that there was only one character: x, = 'a' x, = 'ab' Traceback (most recent call last): File pyshell#29, line 1, in module x, = 'ab' ValueError: too many values to unpack -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
kj wrote:
I just came across an assignment of the form
x, = y
where y is a string (in case it matters).
1. What's the meaning of the comma in the LHS of the assignment?
2. How could I have found this out on my own?
1.Experiment with the interactive interpreter. It is sooo easy.
x, = '1'
x
'1'
x, = '12'
Traceback (most recent call last):
File pyshell#53, line 1, in module
x, = '12'
ValueError: too many values to unpack
x,y = '12'
x,y
('1', '2')
x, = 3
Traceback (most recent call last):
File pyshell#45, line 1, in module
x, = 3
TypeError: 'int' object is not iterable
*x, = [3]
x
[3]
2. Read the Python docs which come with the interpreter.
LanguageReference/SimpleStatements/AssignmentStatements:
If the target list is a comma-separated list of targets:
...
Else: The object must be a sequence with the same number of items as
there are targets in the target list, and the items are assigned, from
left to right, to the corresponding targets.
which is exactly the behavior observed above.
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Re: x, = y (???)
On Thu, 17 Jul 2008 17:32:30 -0400, Terry Reedy [EMAIL PROTECTED] wrote: *x, = [3] x [3] What does *x signify? DaveM -- http://mail.python.org/mailman/listinfo/python-list
Re: x, = y (???)
On Jul 18, 8:01 am, DaveM [EMAIL PROTECTED] wrote: On Thu, 17 Jul 2008 17:32:30 -0400, Terry Reedy [EMAIL PROTECTED] wrote: *x, = [3] x [3] What does *x signify? Mostly that Terry is using Python 3.0. See: http://www.python.org/dev/peps/pep-3132/ -- http://mail.python.org/mailman/listinfo/python-list
Re: x, y coordinates in Tkinter canvas
Let's find out! This program creates a series of rectangles, with the color going from black to nearly white. White corresponds to higher x and y coordinate values. So the answer to your question can be seen in the output of the program. import Tkinter c = Tkinter.Canvas(width=220, height=220) c.pack() for i in range(0, 200, 20): lum = i * 255 / 200 color = #%02x%02x%02x % (lum, lum, lum) c.create_rectangle( i, i, i+40, i+40, fill=color) c.mainloop() pgpSbn6OJyaNx.pgp Description: PGP signature -- http://mail.python.org/mailman/listinfo/python-list
