Re: Executing a command from within python using the subprocess module
In article 5yudnafyso8houtwnz2dnuvz_tidn...@westnet.com.au, R (Chandra) Chandrasekhar chyav...@gmail.com wrote: --- import subprocess width = 5 height = 30 colors = ['#abcdef]', '#456789'] filename = /tmp/image.png # I want to get the equivalent of variable interpolation in Perl # so that the command # # convert -size 5x30 gradient:#abcdef-#456789 /tmp/image.png Here's the equivalent of Peter's cute code in simpler form: cmd = [ 'convert', '-size', '%sx%s' % (width, height), 'gradient:%s-%s' % tuple(colors), # above could also be: 'gradient:%s-%s' % (colors[0], colors[1]), filename, ] subprocess.Popen(cmd) -- Aahz (a...@pythoncraft.com) * http://www.pythoncraft.com/ At Resolver we've found it useful to short-circuit any doubt and just refer to comments in code as 'lies'. :-) -- http://mail.python.org/mailman/listinfo/python-list
Executing a command from within python using the subprocess module
Dear Folks, I want to execute a command from within python using the subprocess module. Coming from a Perl background, I thought I could use variable interpolation in strings, but found that this is neither supported nor the Python way. Accordingly, I am a little at sea about how to accomplish it. I have stated what I am trying to do in the minimal example below: --- import subprocess width = 5 height = 30 colors = ['#abcdef]', '#456789'] filename = /tmp/image.png # I want to get the equivalent of variable interpolation in Perl # so that the command # # convert -size 5x30 gradient:#abcdef-#456789 /tmp/image.png # # is derived from the variables above # and executed by subprocess.call() or subprocess.Popen() # from within Python # # Note that the command convert is from the ImageMagick suite # It exists and is executable by the shell; # the remaining values are arguments. # The command has been confirmed to execute correctly. --- Thanks in advance. Chandra -- http://mail.python.org/mailman/listinfo/python-list
Re: Executing a command from within python using the subprocess module
* R (Chandra) Chandrasekhar:
width = 5
height = 30
colors = ['#abcdef]', '#456789']
filename = /tmp/image.png
# I want to get the equivalent of variable interpolation in Perl
# so that the command
#
# convert -size 5x30 gradient:#abcdef-#456789 /tmp/image.png
#
# is derived from the variables above
Assuming that the extra right square bracket in 'colors' is a typo, here's one
way:
s = convert -size {w}x{h} gradient:{g1}-{g2} {f}.format(
w = width, h = height, g1 = colors[0], g2 = colors[1], f = filename
)
Cheers hth.,
- ALf
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Re: Executing a command from within python using the subprocess module
R (Chandra) Chandrasekhar wrote:
I want to execute a command from within python using the subprocess
module.
Coming from a Perl background, I thought I could use variable
interpolation in strings, but found that this is neither supported nor
the Python way. Accordingly, I am a little at sea about how to
accomplish it.
import subprocess
def convert(width=5, height=30, colors=['#abcdef', '#456789'],
filename=tmp/image with space in its name.png):
lookup = locals()
assert all(\n not in str(s) for s in lookup.values())
subprocess.call(\
convert
-size
{width}x{height}
gradient:{colors[0]}-{colors[1]}
{filename}.format(**lookup).splitlines())
convert()
Peter
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Re: Executing a command from within python using the subprocess module
Peter Otten wrote:
import subprocess
def convert(width=5, height=30, colors=['#abcdef', '#456789'],
filename=tmp/image with space in its name.png):
lookup = locals()
assert all(\n not in str(s) for s in lookup.values())
subprocess.call(\
convert
-size
{width}x{height}
gradient:{colors[0]}-{colors[1]}
{filename}.format(**lookup).splitlines())
convert()
Peter
Thank you. It works. Now I need to understand why and am able to follow
what you are doing part of the way:
1. Assign default values in the function definition.
2. Store the variables existing in the local namespace in the list lookup.
3. Assert that there are no newlines in the values in lookup converted
to strings. (Why? Is this because of splitlines() later on?)
4. Assemble a string (array?) for the subprocess.call argument using the
format string syntax (section 8.1.3 et seq. of the Python
documentation for 2.6.4). Your example works with default option of
shell=False for subprocess.call().
5. I am unable to decipher how you got to format(**lookup).splitlines())
especially the **prefix part, although I guess that splitlines() is
dishing out the arguments one by one from each line in the
subprocess.call argument.
Any correction of (1) to (4) and an explanation of (5) would be most
appreciated.
All in all, your code is a magnificent example of wrapping the function
within python. This is the first time I have seen something like this.
Thank you very much.
Chandra
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Re: Executing a command from within python using the subprocess module
On 2/15/2010 7:35 AM, R (Chandra) Chandrasekhar wrote:
Dear Folks,
I want to execute a command from within python using the subprocess module.
Coming from a Perl background, I thought I could use variable
interpolation in strings, but found that this is neither supported
Yes, it is: see the use of string.Template below.
... nor
the Python way.
That right -- it isn't the Python way. Python offers two basic
alternatives. Alf already presented the use of the new format() method
of string objects. I think traditional Python string formatting might
be what you want in this case:
colors = ['#abcdef', '#456789']
format_string = convert -size 5x30 gradient:%s-%s /tmp/image.png
cmd_string = format_string % tuple(colors)
... or better, by making *colors* a tuple instead of a list ...
colors = ('#abcdef', '#456789')
format_string = convert -size 5x30 gradient:%s-%s /tmp/image.png
cmd_string = format_string % colors
As Peter demonstrated, you need to use split() on *cmd_string* when you
send the command to subprocess.call().
Now if you *really* miss using Perl, try this:
c1, c2 = ('#abcdef', '#456789')
template = string.Template(
convert -size 5x30 gradient:$c1-$c2 /tmp/image.png)
cmd_string = template.substitute(locals())
Not worth the trouble, IMHO.
Refs:
http://www.python.org/doc/2.5.2/lib/typesseq-strings.html
http://www.python.org/doc/2.5.2/lib/node40.html
HTH,
John
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Re: Executing a command from within python using the subprocess module
Peter Otten wrote:
import subprocess
def convert(width=5, height=30, colors=['#abcdef', '#456789'],
filename=tmp/image with space in its name.png):
lookup = locals()
assert all(\n not in str(s) for s in lookup.values())
subprocess.call(\
convert
-size
{width}x{height}
gradient:{colors[0]}-{colors[1]}
{filename}.format(**lookup).splitlines())
convert()
Peter
One other question I forgot to ask is this why is there a terminal
backslash in
subprocess.call(\
Removing the backslash makes the function fail.
I wonder why, because is supposed to allow multi-line strings. I am
sorry if this sounds obtuse but that backslash baffles me.
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Re: Executing a command from within python using the subprocess module
R (Chandra) Chandrasekhar wrote:
Peter Otten wrote:
import subprocess
def convert(width=5, height=30, colors=['#abcdef', '#456789'],
filename=tmp/image with space in its name.png):
lookup = locals()
assert all(\n not in str(s) for s in lookup.values())
subprocess.call(\
convert
-size
{width}x{height}
gradient:{colors[0]}-{colors[1]}
{filename}.format(**lookup).splitlines())
convert()
Peter
One other question I forgot to ask is this why is there a terminal
backslash in
subprocess.call(\
Removing the backslash makes the function fail.
I wonder why, because is supposed to allow multi-line strings. I am
sorry if this sounds obtuse but that backslash baffles me.
It does, but a leading newline would cause the splitlines() result to
start with an empty word - the backslash just causes the interpreter to
ignore the newline immediately following, rather than including it in
the string literal's value.
regards
Steve
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Re: Executing a command from within python using the subprocess module
Alf P. Steinbach wrote:
* R (Chandra) Chandrasekhar:
width = 5
height = 30
colors = ['#abcdef]', '#456789']
filename = /tmp/image.png
# I want to get the equivalent of variable interpolation in Perl
# so that the command
#
# convert -size 5x30 gradient:#abcdef-#456789 /tmp/image.png
#
# is derived from the variables above
Assuming that the extra right square bracket in 'colors' is a typo,
here's one way:
s = convert -size {w}x{h} gradient:{g1}-{g2} {f}.format(
w = width, h = height, g1 = colors[0], g2 = colors[1], f = filename
)
Cheers hth.,
- ALf
Thanks, Alf. It works if I use it so:
subprocess.call(s, shell=True)
and I think that is because s is a single string assembled in almost the
variable interpolation fashion of Perl. It does not work with the
default shell=False argument, presumably because the arguments are not
split into separate strings (something that was horrible to do by hand
using the %s and %() syntax that I had tried out previously).
I think that you and Peter have, between you, shown me two ways of using
subprocess.call(): one with shell=True and the other with shell = False.
Thanks.
Chandra
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Re: Executing a command from within python using the subprocess module
R (Chandra) Chandrasekhar wrote:
Peter Otten wrote:
import subprocess
def convert(width=5, height=30, colors=['#abcdef', '#456789'],
filename=tmp/image with space in its name.png):
lookup = locals()
assert all(\n not in str(s) for s in lookup.values())
subprocess.call(\
convert
-size
{width}x{height}
gradient:{colors[0]}-{colors[1]}
{filename}.format(**lookup).splitlines())
convert()
Peter
Thank you. It works. Now I need to understand why and am able to follow
what you are doing part of the way:
1. Assign default values in the function definition.
2. Store the variables existing in the local namespace in the list lookup.
3. Assert that there are no newlines in the values in lookup converted
to strings. (Why? Is this because of splitlines() later on?)
Yes.
4. Assemble a string (array?) for the subprocess.call argument using the
format string syntax (section 8.1.3 et seq. of the Python
documentation for 2.6.4). Your example works with default option of
shell=False for subprocess.call().
I wanted to pass the executable and its arguments as a list (a python list
is called array in other languages) to avoid bothering with shell escapes.
5. I am unable to decipher how you got to format(**lookup).splitlines())
especially the **prefix part, although I guess that splitlines() is
dishing out the arguments one by one from each line in the
subprocess.call argument.
Given some_dict = {key1: value1, key2: value2,...}
f(**some_dict)
is a shortcut for
f(key1=value1, key2=value2, ...)
with the additional twist that you do not have to know the key/value pairs
in advance.
I introduced splitlines to avoid calling the format method on every argument
to convert, i. e.
def convert2(width=5, height=30, colors=['#abcdef', '#456789'],
filename=tmp/image with space in its name.png):
subprocess.call([
convert,
-size,
{width}x{height}.format(width=width, height=height),
gradient:{0}-{1}.format(*colors),
filename])
I supected that it would look cleaner than the above, but now that I tried
it I think convert2() is the better alternative.
Peter
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Re: Executing a command from within python using the subprocess module
On Tue, 16 Feb 2010 00:11:36 +0800, R (Chandra) Chandrasekhar wrote: One other question I forgot to ask is this why is there a terminal backslash in subprocess.call(\ Removing the backslash makes the function fail. I wonder why, because is supposed to allow multi-line strings. I am sorry if this sounds obtuse but that backslash baffles me. The backslash causes the following newline to be skipped, so the c at the beginning of convert is the first character in the string. Without it, the newline would be the first character, and the .splitlines() would result in an empty string as the first element in the list. Example: = hello = world = .splitlines() ['', 'hello', 'world'] \ = hello = world = .splitlines() ['hello', 'world'] -- http://mail.python.org/mailman/listinfo/python-list
