How Do I Determine indentation in AST?

[jimo]

I am new to Python though I have many, many years of programming experience. Using Python 3.6.5 on Fedora 28, I am investigating automatic generation of docstrings. To start with, I have created the following script just to work out some of the details:

import ast
import astor
import inspect

source = '''
class cl:
    """This is doc for cl class"""

    pass'''

tree = ast.parse(source)
for node in ast.iter_child_nodes(tree):
    if isinstance(node, ast.ClassDef):
        body = node.body[0]
        body.value.s += "\n\n    More documentation."
        doc = ast.get_docstring(node)
        print(doc)
        print()

src = ast.dump(tree)
print(src)
print()
newsource = astor.to_source(tree)
print(newsource)
print()
newSrc = inspect.cleandoc(newsource)
print(newSrc)

At line 15, I am adding text to the docstring for the class and have included 4 spaces at the beginning of the text because that fits the existing source code. How do I determine the indentation in general?

Here is the output from running the script:

Output:
This is doc for cl class

More documentation.

Module(body=[ClassDef(name='cl', bases=[], keywords=[], body=[Expr(value=Str(s='This is doc for cl class\n\n    More documentation.')), Pass()], decorator_list=[])])

class cl:
    """This is doc for cl class

    More documentation."""
    pass


class cl:
"""This is doc for cl class

More documentation."""
pass