What does this mean in python?

Question

I am having difficulty in understand some code in python and would really appreciate any help on it.

condition = "{field}::{cast} {operator} {value}".format(**{
    'field': field_exp(field),
    'cast': cast,
    'value': json.dumps(parameter).replace('"', "'"),
    'operator': operator})

I cannot understand what's really happening in this statement. what does the "double colon" do? And what does format(**... mean?

Further in the code there is another line:

condition = "jsonb_typeof({field}::jsonb) is {not} null".format(**{
    'field': field_exp(field),
    'not': 'not' if parameter else ''})

This statement again follows similar pattern. I will really appreciate if someone could explain what's going on here.

Thank you, Asanas

The double colon doesn't do anything special. It just means the resulting string will have two colons in it, the same way your second string will have "jsonb_typeof" and "is" and "null" in it. — Kevin, Feb 18 '16 at 14:30

score 0 · Accepted Answer · answered Feb 18 '16 at 14:35

The double colon doesn't do anything. It's just a fixed part of the format string.

The ** notation allows keyword arguments to be specified from a dict. Here's a simple, similar example that should make this clear. I assigned the dict to a variable rather than using it in-line to make it more obvious what's happening:

>>> vals = {"a":"w", "b":"x", "c":"y", "d":"z"}
>>> "{a}::{b} {c} {d}".format(**vals)
'w::x y z'
>>>

As you can see, format is just substituting the values from its keyword arguments, which are given by vals.

What does this mean in python?

1 Answers1