Python 2.7 assertion error (noob needs help)

Question

I'm fairly new to programming. I'm trying to write two class methods that will take a string, '{{name}} is in {{course}}' , and replace {{name}} and {{course}} with their respective Key values in a dictionary. So:

t = Template()
vars = {
    'name': 'Jane',
    'course': 'CS 1410'
    }

out = t.process('{{name}} is in {{course}}', vars)
print 'out is: [' + out + ']'

Would print:

Jane is in CS 1410

My code goes as:

class Template:

    def processVariable(self, template, data):

        print template
        assert(template.startswith('{{'))

        start = template.find("{{")
        end = template.find("}}")
        out = template[start+2:end]

        assert(out != None)
        assert(out in data)

        return data[out]

    def process(self, template, data):

        output = ""
        check = True

        while check == True:
            start = template.find("{{")
            end = template.find("}}")
            output += template[:start]
            output += self.processVariable(template[start:end+2], data)
            template = template.replace(template[:end+2], "")
            for  i in template:
                if i == "}}":
                    check = True 

        output += template

        return output

t = Template()
vars = {
    'name': 'Jane',
    'course': 'CS 1410'
    }

out = t.process('{{name}} is in {{course}}', vars)
print 'out is: [' + out + ']'

When I run the code, I get the following output:

{{name}}
{{course}}

Traceback (most recent call last):
  File "C:some/filepath/name.py", line 46, in <module>
    out = t.process('{{name}} is in {{course}}', vars)
  File "C:some/filepath/name.py", line 28, in process
    output += self.processVariable(template[start:end+2], data)
  File "C:some/filepath/name.py", line 8, in processVariable
    assert(template.startswith('{{'))
AssertionError

I just don't understand why im getting that assertion error if template is '{{course}}' Edit: The purpose making the code this way, was to bring in any dictionary and string, so that I can create a simple social network. Otherwise much simpler methods would be proficient.

Answer 1

You weren't actually getting the assertion error when template was {{course}}, which you can see for yourself if you change the process method to include some simple print statements, e.g.:

def process(self, template, data):
        # ...
        output += template[:start]
        print "Processing, template is currently:"
        print template
        output += self.processVariable(template[start:end+2], data)
        # ...

    return output

The actual problem was that check never became false. You can replace your if test with something like this, and then your function runs fine:

if not '}}' in template:
    check = False 

Answer 2

Marius beat me to the answer to your question, but I just wanted to point out an easier way to do (almost) the same thing. Ofcourse, if you're just trying to learn than the hard way is usually better.

vars = {
    'name': 'Jane',
    'course': 'CS 1410'
    }

out = '{name} is in {course}'.format(**vars)
print out