Modify python file as if it were a text file, but still having variables and such

Question

I am testing some python stuff, and i started wondering if it was possible to modify a python file as if it were a text file.

I have looked at the open(<filename>, "r+"), but this just edits the file. I can't modify it like it was a python script, modifying the actual script and not just the cache.

---

#main.py

import filea

filea.var1edit("Hello world")

Expected before:

#filea.py

var1 = "Text"
def var1edit(var):
  global var1
  var1 = var

Expected after:

#filea.py

var1 = "Hello world"
def var1edit(var):
  global var1
  var1 = var

---

Actual after:

#filea.py

var1 = "Text"
def var1edit(var):
  global var1
  var1 = var

Answer 1

Module ast and astunparse can be useful. We first read the .py file, generate the AST, modify the AST, unparse it and write the new code back. Here is an example:

#main.py
import ast
import astunparse

class ReWriteSource(ast.NodeTransformer):
    #visit 'Assign' node and modify
    def visit_Assign(self,node):
        if node.targets[0].id == 'var1' and isinstance(node.value,ast.Str):
            node.value.s = 'Hello World!'
        return node

#read .py file
with open('filea.py','r') as f:
    source = f.read()
#generate the AST
source_ast = ast.parse(source)
#modify the AST
ReWriteSource().visit(source_ast)
#unparse and write it back
with open('filea.py','w') as f:
    f.write(astunparse.unparse(source_ast))

original filea.py:

#filea.py
var1 = 'Text'
def var1edit(var):
    global var1
    var1 = var

modified filea.py:

#filea.py
var1 = 'Hello World!'
def var1edit(var):
    global var1
    var1 = var