Python: Determine String Formatting With a boolean?

Question

I'm new to Python and I am wondering what the best way is to format a string with the parts inserted determined by booleans.

I have a long html form string that can have error messages. I only want to show error messages for those parts of the form that did not validate correctly:

   form = """Signup
    <form method="post">
      <table>
        <tr>
          <td class="label">
            Username
          </td>
          <td>
            <input type="text" name="username" value="%(un)s">
          </td>
          <td class="error"> 
              %(un_err)s
          </td>
        </tr>
       ............

    </form>"""

When I am processing the post call I have this:

self.response.out.write(form % {"un": username, "pw": password, "pwv": validate, "email": email, "un_err": "", "pw_err": "", "verify_err": "", "email_err": ""})

Each "..._err" message is determined by a separate boolean, for example, if valid_username is false then the output should be:

self.response.out.write(form % {"un": username, "pw": password, "pwv": validate, "email": email, "un_err": "This is not a valid username", "pw_err": "", "verify_err": "", "email_err": ""})

Is there a nice way to do this?

Jesse · Accepted Answer · 2012-05-01 18:11:43Z

You've reached the point where moving to a templating system would be beneficial. It looks like you're using Google AppEngine, which has support for Django and Jinja2 templates. You can find more info regarding AppEngine's specific integration in their documentation.

I recommend Jinja2 templates, which could make it easier to achieve your goals. Your template could contain something like:

<form method="post">
  <table>
    <tr>
      <td class="label">
        Username
      </td>
      <td>
        <input type="text" name="username" value="%(un)s">
      </td>
      <td class="error"> 
         {% if un_err %}
           {{ un_err }}
         {% endif %}
      </td>
    </tr>
    ...
  </table>
</form>

And when you're processing the page for rendering you would be able to do something similar to:

template = jinja_environment.get_template('signup_form.html')
self.response.out.write(template.render(template_values))

Ignacio Vazquez-Abrams · Accepted Answer · 2012-05-01 17:59:27Z

1

un_err_map = {
  True: "",
  False: "This is not a valid username"
}

 ...

self.response.out.write(form % {"un": username, "pw": password, "pwv": validate, "email": email, "un_err": un_err_map[valid_username], "pw_err": "", "verify_err": "", "email_err": ""})

answered May 1, 2012 at 17:59

Ignacio Vazquez-Abrams

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Comments

Israel Unterman · Accepted Answer · 2012-05-01 18:02:18Z

1

Use this syntax:

"un_err": "" if valid_username else "This is not a valid username", ...

If it feels to long you can make it into a function:

def bool_it(var, s):
    return "" if var else s

and then:

"un_err": bool_it(valid_username, "This is not a valid username")

answered May 1, 2012 at 18:02

Israel Unterman

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