call a function from other/Program python

You're opening the file in a for loop, and with the w as the mode parameter for the open function it makes open overwrite the file it opens, which is why you only get the output from the last iteration of the loop.

You should open the file outside the loop instead:

with open("input.txt", "w") as input_file:
    for i in list:
        print("{}".format(i), file = input_file)

In first.py, change your code like this.

w mode is for write operation. In each iteration of for loop you are overwriting the last content and writing the new one. So input.txt was having ef in that (finally).

list=["ab","cd","ef"]

for i in list:
    with open("input.txt", "a+") as input_file:
        print("{}".format(i), file = input_file)

And now you will get what you expected from this. Now input.txt will have the following unlike with your case.

ab
cd
ef

Note: But if you will will run first.py 2nd time, it will continue to add as a+ creates file if file does not exist otherwise it appends. For better working of this code use os.path module's exists() function.

And if you want to call the code available in first.py then wrap it inside the function. Then import that function in second.py and call.

For example

First make sure first.py and second.py are in the same directory.

first.py

def create_file(file_name):
    list=["ab","cd","ef"]
    for i in list:
        with open(file_name, "a+") as input_file:
            print(" {}".format(i), file = input_file)

second.py

from first import create_file

def read_file(file_name):
    # Create file with content
    create_file(file_name)

    # Now read file content
    input_file = open(file_name, 'r')     
    for line in input_file:
        if "ef" in line:
           print(line)

read_file('input.txt')

Open terminal, navigate to this directory, run python second.py.

https://www.learnpython.org/en/Module... | https://www.digitalocean.com... | https://www.programiz.com/pytho... would be the helpers for you if you want to read and try how to create module/package in Python.

Update: The above has a problem as you have mentioned in comment, in each each run, it will append the content. Let's fix it with a little change to first.py as follows.

import os

def create_file(file_name):
    l = ["ab", "cd", "ef"]

    if os.path.exists(file_name): # If file `input.txt` exists (for this example)
        os.remove(file_name)      # Delete the file

    for i in l:
        with open(file_name, "a+") as input_file:
            print(" {}".format(i), file = input_file)

That is it (update in comment if you are stuck).