如何使用 python 修改文件内容？

Question

The following code reads a file, uses syntax tree to append fullstop to docstrings of that file. How can I save changes made in called file? I understand present code doesn't change content in original file but the local variables accessing it. Can you suggest changes, if possible provide learning resource as well?

astcode.py

import ast
import sys
import os

filename = sys.argv[1]


# """Getting all the functions"""
ast_filename = os.path.splitext(ast.__file__)[0] + '.py'

with open(filename) as fd:
    file_contents = fd.read()

module = ast.parse(file_contents)

# with open(filename, 'w') as file:
# module level
if(isinstance(module.body[0], ast.Expr)):
    docstr = module.body[0].value.s
    if(module.body[0].value.s not in '.'):
        docstr += '.'
        # ast.dump(module, include_attributes=True)
    print(docstr)


# function level
function_definitions = [node for node in module.body if isinstance(node, ast.FunctionDef)]

for function in function_definitions:
    # next_node = function_definitions[idx].body
    next_node = function.body
    for new_node in next_node:
        if(isinstance(new_node, ast.Expr)):
            if(isinstance(new_node.value, ast.Str)):
                # Docstring stored in docstr variable.
                docstr = new_node.value.s

                if(docstr[-1] not in '.'):
                    new_node.value.s += '.'
                
                # astString = ast.dump(new_node, annotate_fields=True, include_attributes=True)
                # print(astString)
                # compile(astString, filename, 'eval')
                # print(exec(astString))
                print(new_node.value.s)

    # for line in module:
        # file.write(line)

Example

testfile.py

def readDictionaryFile(dictionary_filename):
    """readDictionaryfile doc string"""
    return []

def readTextFile(text_filename):
    """readTextfile doc string"""
    return []

$ python3 astcode.py testfile.py

Expected

testfile.py

def readDictionaryFile(dictionary_filename):
    """readDictionaryfile doc string."""
    return []

def readTextFile(text_filename):
    """readTextfile doc string."""
    return []

Note: Fullstop(.) appended.

Answer 1

Looking at the documentation link, I notice there's a NodeVisitor and NodeTransformer with a code example. I looked at how they unparse a function def, and it's basically the same as you've done in your original question, so I used that.

# https://docs.python.org/3/library/ast.html#ast.NodeTransformer
class MyDocstringTransformer(ast.NodeTransformer):
    def visit_FunctionDef(self, node):
        if len(node.body):
            if isinstance(node.body[0], ast.Expr):
                if isinstance(node.body[0].value, ast.Constant):
                    if isinstance(node.body[0].value.value, str):
                        docstring = node.body[0].value.value
                        node.body[0].value.value = docstring + '.'
        return node

Using python 3.9's ast module gets us https://docs.python.org/3/library/ast.html#ast.unparse which is about as close as we can get to changing the ast node and then rewriting the original file.

tree = ast.parse(file_contents)
new_tree = MyDocstringTransformer().visit(tree)
print(ast.unparse(new_tree))

Instead of just overwriting to the same filename, you may want to write to a temp file, then let the OS attempt to delete the old file and rename the temp file to the old name, thus performing the replace in the OS.