Python: Why is this string invalid?

Question

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I have this string:

'  ServerAlias {hostNameshort}.* www.{hostNameshort}.*'.format(hostNameshort=hostNameshort)

But it keeps giving me a syntax error. The line is supposed to be this bash equivalent:

echo "    ServerAlias ${hostOnly}.* www.${hostOnly}.*" >> $prjFile

Mind you the first string is a part of a myFile.write function but that isn't the issue, I can't even get the string to make enough sense for it to let me run the program.

Traceback:

  File "tomahawk_alpha.py", line 89
    '  ServerAlias {hostNameshort}.* www.{hostNameshort}.*'.format(hostNameshort=hostNameshort)
                                                          ^

But no matter how I change that ' symbol it doesn't seem to work. What am I doing wrong?

In response to @mgilson:

    myFile = open(prjFile, 'w+')
    myFile.write("<VirtualHost 192.168.75.100:80>"
                 "  ServerName www.{hostName}".format(hostName=hostName)
                 '  ServerAlias {hostNameshort}.* www.{hostNameshort}.*'.format(hostNameshort=hostNameshort)
                 "  DocumentRoot ", prjDir, "/html"
                 '  CustomLog "\"|/usr/sbin/cronolog /var/log/httpd/class/',prjCode,'/\{hostName}.log.%Y%m%d\" urchin"'.format(hostName=hostName)
                 "</VirtualHost>")
    myFile.close()

I had each line in it's own myFile.write line, but it only produced the first line and then quit. So I assumed calling it once and spacing it like that would create intended result.

I don't see anything wrong with the string. Are all your parenthesis closed on the previous line? — mgilson
– mgilson, Commented May 31, 2013 at 17:32
In your traceback it looks like you removed the error type. Can you include this so we can better diagnose the problem? — SethMMorton
– SethMMorton, Commented May 31, 2013 at 17:33
The problem might be on some the previous line as mgilson mentioned. You chould also check that all the ' are closed as well. — SethMMorton
– SethMMorton, Commented May 31, 2013 at 17:34

mgilson · Accepted Answer · 2013-05-31 17:45:24Z

4

Automatic string concatenation only works with string literals:

"foo"  "bar"

results in "foobar"

But, the following won't work:

("{}".format("foo") 
 "bar")

which is analogous to what you are doing. The parser sees something like this:

"{}".format("foo") "bar"

(because it joins lines where there are unterminated parenthesis) and that is clearly not valid syntax. To fix it, you need to concatenate the strings explicitly. e.g:

("{}".format("foo") +
 "bar")

Or use string formatting on the entire string, not just one piece of it at a time.

edited May 31, 2013 at 17:45

answered May 31, 2013 at 17:37

mgilson

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Sean Vieira · Accepted Answer · 2013-05-31 17:39:39Z

3

You have a couple of syntax errors. However, you may want to look at using a triply quoted strings instead - much easier to modify in the long run:

myFile.write("""<VirtualHost 192.168.75.100:80>
               ServerName www.{hostName}
               ServerAlias {hostNameshort}.* www.{hostNameshort}.*
               DocumentRoot {prjDir}/html
               CustomLog "\"|/usr/sbin/cronolog /var/log/httpd/class/{prjCode}/\{hostName}.log.%Y%m%d\" urchin"
             </VirtualHost>""".format(hostName=hn, hostNameshort=hns, prjDir=prjd, prjCode=prjc))

answered May 31, 2013 at 17:39

Sean Vieira

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1 Comment

user2433340 Over a year ago

This works wonderfully, and I am going to use this triply quoted system with .format more often. Very nice!

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