What does ( (value_in_dictionary) modulo[%] (dictionary) ) does in Python?

Question

I want to understand what value_in_dictionary % dictionary does.

I tried

values = {
    'hello':'world',
    'hello2':'world2',
}
value = 'world'
print(value % values)

This prints

world

Answer 1

For strings, modulo triggers old style string interpolation. The right hand operand can be a scalar, tuple or dictionary.

The documentation reads as follows:

When the right argument is a dictionary (or other mapping type), then the formats in the string must include a parenthesised mapping key into that dictionary inserted immediately after the '%' character. The mapping key selects the value to be formatted from the mapping. For example:

>>> print('%(language)s has %(number)03d quote types.' %
...       {'language': "Python", "number": 2})
Python has 002 quote types.

Your particular example has no formats in it, so nothing gets interpolated. Adding a valid interpolation key like world = 'hello %(hello)' or world = 'hello %(hello2)' would illustrate how it works. Attempting world = 'hello %(hello3)' would result in a KeyError.