Django store regular expression in DB which then gets evaluated on page

Question

Hi,

I want to store a number of url patterns in my django model which a user can provide parameters to which will create a url.

For example I might store these 3 urls in my db where %s is the variable parameter provided by the user:

• www.thisissomewebsite.com?param=%s
• www.anotherurl/%s/
• www.lastexample.co.uk?param1=%s&fixedparam=2

As you can see from these examples the parameter can appear anywhere in the string and not in a fixed position.

I have 2 models, one holds the urls and one holds the variables:

class URLPatterns(models.Model):
    pattern = models.CharField(max_length=255)

class URLVariables(models.Model):
    pattern = models.ForeignKey(URLPatterns)
    param = models.CharField(max_length=255)

What would be the best way to generate these urls by replacing the %s with the variable in the database.

would it just be a simple replace on the string e.g:

urlvariable = URLVariable.objects.get(pk=1)
pattern = url.pattern
url = pattern.replace("%s", urlvariable.param)

or is there a better way?

EDIT It would also be nice if the user could choose to store either a single variable or a list of variables which would then replace a number of variables in the string e.g.

u = URLPatterns(pattern='www.url?param=%s&param2=%s&param3=%s')
v = URLVariables(pattern=u, param='[2,6,3]')

url = SOME WAY TO REPLACE THE 3 %s WITH THE 3 VARIABLES IN THE ARRAY (this would need to be converted from a string in someway)

Thanks

Answer 1

You can do this:

pattern="www.url?param=%s&param2=%s&param3=%s"
params = "[1, 2, 3]"

url = pattern % tuple(s.strip('[] ') for s in params.split(','))

or use ast.literal_eval(params) as Ignacio suggests.

Answer 2

>>> param = [2, 6, 3]
>>> pattern = 'www.url?param=%s&param2=%s&param3=%s'
>>> url = pattern % tuple(param)
>>> url
'www.url?param=2&param2=6&param3=3'

if param is a string like '[2,6,3]' you can use ast.literal_eval() or json.loads():

>>> ast.literal_eval('[2,6,3]')
[2, 6, 3]

or

>>> json.loads(param)
[2, 6, 3]

or

>>> simplejson.loads(param)
[2, 6, 3]

Answer 3

ast.literal_eval() can be used to parse a string into a Python value or structure. If it's a list then just pass it to tuple() before using string interpolation.