When trying to run a script containing the following code for generating text block:

from textwrap import dedenttext = dedent("""\yada yada '1' ('2','3',4') ('{0}', Null, '{1}','{"Hello":"world",}', '1', '{2}');""").format("yada1","yada2","yada3")

I get consistent error KeyError '\n "Hello"
and trace back pointing at the line of the .format().

When I remove the format everything is ok, but I need it to enter parameters dynamically.
(Originally its reside inside a loop)

You need to double the { and } characters that are not placeholders:

text = dedent("""\yada yada '1' ('2','3',4') ('{0}', Null, '{1}','{{"Hello":"world",}}', '1', '{2}');""").format("yada1","yada2","yada3")

otherwise Python sees a {\n "Hello":"world",\n} placeholder, where the part up to the : is the placeholder name.

From the Format String Syntax documenattion:

Format strings contain “replacement fields” surrounded by curly braces {}. Anything that is not contained in braces is considered literal text, which is copied unchanged to the output. If you need to include a brace character in the literal text, it can be escaped by doubling: {{ and }}.

(emphasis mine).