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How to prevent reload from disk when using open() in Python?

This is my first post on StackOverflow. I'm also a beginner in Python. So I was just tinkering with the open() function, I was making a simple program to replace text in other .txt files. Here is my code:
f = open("file.txt", "r+")
f.truncate(0)
f.write("This text has been replaced.")
f.close()
print("Text replaced")
So, after running this program, the text in "file.txt" is getting changed. However, when I do ctrl + z, it's showing Undo Reload from Disk?, and when you click OK, the text gets back to normal.
How to prevent this? I am using Python 3.9, Pycharm code editor.
Thank you

I think your question might be more about the PyCharm UI editor.
If I understand correctly:
you have "file.txt" open in the PyCharm editor
you run the code above
At this point, the filesystem has the "file.txt" with the updated contents. The PyCharm editor (buffer) still has the old contents.
When you ctrl+z, PyCharm notices the filesystem has been updated and prompts to see how you want to proceed. When you click "OK", PyCharm writes its buffer to the file, which is the original contents.

Get text from a program in Python

I would like to get the text from a program with Python, for example from notepad. Can I "request" this text, just like from a website? I thought about something like this:
A document in Notepad:
Hello World!
This is a text!
GetText.py:
get_text("notepad.exe")
>>> Hello World!\nThis is a text!
Is this possible?

No, you can't, not directly.
There are various accessibility, etc. APIs you could use to try and "read" the user interface of another program, but that's certainly a lot more involved than just a simple get_text() style call.
(And for Windows Notepad, you can enumerate the Notepad main window's child windows, find the edit/rich-text control it's using and send a WM_GETTEXT message (if my memory serves) and hope it sends you some of the current text back...)

You can open the file in read-mode and simply print each line of the file using a for loop:
a_file = open('notepad.exe', 'r')
for line in a_file:
print(line)
a_file.close() #Make sure you close whatever file you open
If you are using Jupyter Notebook, make sure notepad.exe is in the same directory as you have your notebook opened in.
Side note: If you have experience with the command line (e.g. Linux), you can also open it in a text editor like vim. There, you can more readily see and edit it.

Python Clipboard Assistent (Windows)

I am trying to create a tool that assists me in filling out a specific web form.
For that I have a „.txt“ with the necessary information. Now I am trying to find a python module that detects that I pasted the last content and loads the next content (from txt) into the clipboard. Is that possible with python?

I believe you can the pyperclip module to copy and paste text to and from the clipboard. You can take a look at http://pyperclip.readthedocs.io/en/latest/introduction.html for more info.
They have an example where they copy a string onto a clipboard and paste it.
>>> import pyperclip
>>> pyperclip.copy('Hello world!')
>>> pyperclip.paste()
'Hello world!'
But of course, I believe you can extract the string from your text file and use it.

How to use the Windows Explorer or Finder file dialogues in Tkinter/Python2.7.3?

Among other things, I am currently trying to create a basic text editor which can open text files, edit them, and then save them. I have used this Tkinter dialogue for the GUI 'file manager,' but I was wondering if anyone knew the way to access the one that comes default on Windows?
Thanks!
Technical Things:
OS: Windows 7
Language: Python 2.7.3
EDIT 1
By the DEFAULT file dialogue, I mean the windows explorer dialogue:
I also use mac. Assuming that my application is cross-platform, would there be any way for me to have the program check what the os was, and then open either Finder or Windows Explorer.
I need the program to be able to save and open items in different commands. How would I do this?

It's not exactly clear what you're asking, since the one that tkinter comes with is default in Windows. Here's another link for that, just in case you got mixed up somewhere along the line. Remember that you can set it so it only finds a certain type of file, starts in a specific place, returns the filename or directory, or even open the file (I think)
If you mean the Windows Explorer you can open it and close it with pywin32, but not much else. Taken from this answer
import subprocess
subprocess.Popen(r'explorer /select,"C:\path\of\folder\file"')

try importing tkFileDialog:
import tkFileDialog as tkfd
def save():
savenm = tkfd.asksaveasfile()
f = open(savenm.name,"w")
# then put what to do with the opened file
def open():
opennm = tkfd.askopenfile()
f = open(savenm.name,"r")
# then put what to do with the opened file
then make a button that uses the functions:
import Tkinter as tk
root=tk.Tk()
SAVELOADFRAME = tk.Frame(root)
SAVELOADFRAME.pack()
savebtn = Button(SAVELOADFRAME,text="Save",command=save)
savebtn.pack(side=LEFT)
root.mainloop()
loadbtn = Button(SAVELOADFRAME,text="Open",command=open)
loadbtn.pack(side=RIGHT)
maybe if you have a notepad box you might want to insert the text from the file into the tk.Text widget. The above code only works for text based files really (e.g. *.js, *.txt, *.py) not *.exe, *.dll, etcetera.
hope that solves your problem :^)

Multiple lines user input in command-line Python application

Is there any easy way to handle multiple lines user input in command-line Python application?
I was looking for an answer without any result, because I don't want to:
read data from a file (I know, it's the easiest way);
create any GUI (let's stay with just a command line, OK?);
load text line by line (it should pasted at once, not typed and not pasted line by line);
work with each of lines separately (I'd like to have whole text as a string).
What I would like to achieve is to allow user pasting whole text (containing multiple lines) and capture the input as one string in entirely command-line tool. Is it possible in Python?
It would be great, if the solution worked both in Linux and Windows environments (I've heard that e.g. some solutions may cause problems due to the way cmd.exe works).

import sys
text = sys.stdin.read()
After pasting, you have to tell python that there is no more input by sending an end-of-file control character (ctrl+D in Linux, ctrl+Z followed by enter in Windows).
This method also works with pipes. If the above script is called paste.py, you can do
$ echo "hello" | python paste.py
and text will be equal to "hello\n". It's the same in windows:
C:\Python27>dir | python paste.py
The above command will save the output of dir to the text variable. There is no need to manually type an end-of-file character when the input is provided using pipes -- python will be notified automatically when the program creating the input has completed.

You could get the text from clipboard without any additional actions which raw_input() requires from a user to paste the multiline text:
import Tkinter
root = Tkinter.Tk()
root.withdraw()
text = root.clipboard_get()
root.destroy()
See also How do I copy a string to the clipboard on Windows using Python?

Use :
input = raw_input("Enter text")
These gets in input as a string all the input. So if you paste a whole text, all of it will be in the input variable.
EDIT: Apparently, this works only with Python Shell on Windows.