KeyError in template when key is definetly present

Question

I want to render in template data like that: [{'name': 'Some name', 'description': 'Description for this item'}]

I try to use this code:

{% for item in source_data %}
    <tr>
        <td>{{ item['name'] }}</td>
        <td>{{ item['description'] }}</td>
    </tr>
{% end %}

It doesn't work, because I receive KeyError: 'description' exception.

But if I change second placeholder to {{ item.get('description') }} it works as expected (Correct data from dictionary was printed, not default value).

What can produce this kind of error?

Answer 1

Looks like not all of your dictionaries have description key.

Instead of directly getting the value by key, use dictionary get() method, that won't throw KeyError if a key wasn't found:

{% for item in source_data %}
    <tr>
        <td>{{ item['name'] }}</td>
        <td>{{ item.get('description', 'No description') }}</td>
    </tr>
{% end %}

Demo:

>>> from tornado import template
>>> source_data = [
...     {'name': 'Some name', 'description': 'Description for this item'},
...     {'name': 'test'}
... ]

>>> template1 = template.Template("""
... {% for item in source_data %}
...     {{ item['description'] }}
... {% end %}
... """)

>>> template2 = template.Template("""
... {% for item in source_data %}
...     {{ item.get('description', 'No description found') }}
... {% end %}
... """)

>>> print template1.generate(source_data=source_data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "tornado/template.py", line 278, in generate
    return execute()
  File "<string>.generated.py", line 7, in _tt_execute
KeyError: 'description'
>>> print template2.generate(source_data=source_data)

    Description for this item

    No description found

Hope that helps.