Basic text editor in Python with Tkinter
Style

Python has a guide style called PEP 8. If you have no good reason not to follow it, just follow it. You'll find various tools to check your code (pep8, pep8online.com/, etc) and even to fix it more or less automatically (autopep8).

Here's a list of problems detected :

txt_original.py:13:80: E501 line too long (105 &gt; 79 characters)
txt_original.py:14:80: E501 line too long (105 &gt; 79 characters)
txt_original.py:15:80: E501 line too long (141 &gt; 79 characters)
txt_original.py:17:80: E501 line too long (138 &gt; 79 characters)
txt_original.py:22:43: E231 missing whitespace after ','
txt_original.py:25:8: E713 test for membership should be 'not in'
txt_original.py:30:35: E231 missing whitespace after ','
txt_original.py:33:43: E231 missing whitespace after ','
txt_original.py:35:1: E302 expected 2 blank lines, found 1
txt_original.py:37:37: E231 missing whitespace after ','
txt_original.py:40:8: E713 test for membership should be 'not in'
txt_original.py:43:23: E231 missing whitespace after ','
txt_original.py:44:34: E231 missing whitespace after ','
txt_original.py:45:46: E231 missing whitespace after ','
txt_original.py:45:80: E501 line too long (95 &gt; 79 characters)
txt_original.py:47:1: E302 expected 2 blank lines, found 1
txt_original.py:55:1: E302 expected 2 blank lines, found 1
txt_original.py:61:56: E231 missing whitespace after ','
txt_original.py:61:65: W291 trailing whitespace
txt_original.py:68:42: E231 missing whitespace after ','
txt_original.py:69:37: E231 missing whitespace after ','
txt_original.py:75:21: E231 missing whitespace after ','
txt_original.py:76:14: E251 unexpected spaces around keyword / parameter equals
txt_original.py:76:16: E251 unexpected spaces around keyword / parameter equals
txt_original.py:76:20: E231 missing whitespace after ','
txt_original.py:79:20: E251 unexpected spaces around keyword / parameter equals
txt_original.py:79:22: E251 unexpected spaces around keyword / parameter equals
txt_original.py:79:26: E231 missing whitespace after ','


And here's you code after most of the problems are fixed :

try:
    import tkinter as tk
except:
    import Tkinter as tk

try:
    import tkMessageBox as pop_up
except:
    import tkinter.tkMessageBox as pop_up

import time

EMPTY_TITLE_ERROR_MESSAGE_SAVE = "Please write the name of the file you want to save in the given field."
EMPTY_TITLE_ERROR_MESSAGE_OPEN = "Please write the name of the file you want to open in the given field."
FILE_NOT_FOUND_ERROR_MESSAGE = "No file with the given title was found, remember that this text editor can only read files in its directory."
SAVING_SUCCESS_MESSAGE = "Your text is now stored in the {filename} file"
SIGNATURE_TXT_NOT_FOUND_MESSAGE = "Please be sure that the file you want to open exists and that it is in the same folder of this editor."


def _open():
    if not file_title.get():
        pop_up.showerror("Title is empty.", EMPTY_TITLE_ERROR_MESSAGE_OPEN)
        return 1

    if ".txt" not in file_title.get():
        filename = file_title.get() + ".txt"

    try:
        with open(filename) as f:
            main_text.delete("1.0", tk.END)
            main_text.insert(tk.INSERT, f.read(), "a")
    except IOError:
        pop_up.showerror("File not found.", FILE_NOT_FOUND_ERROR_MESSAGE)


def save():
    if not file_title.get():
        pop_up.showerror("No title.", EMPTY_TITLE_ERROR_MESSAGE_SAVE)
        return 1

    if ".txt" not in file_title.get():
        filename = file_title.get() + ".txt"

    with open(filename, "w+") as f:
        f.write(main_text.get(1.0, tk.END))
    pop_up.showinfo(
        "File saved succesfully.",
        SAVING_SUCCESS_MESSAGE.format(
            filename=filename))


def add_date():
    full_date = time.localtime()
    day = str(full_date.tm_mday)
    month = str(full_date.tm_mon)
    year = str(full_date.tm_year)
    date = "\n" + day + '/' + month + '/' + year
    main_text.insert(tk.INSERT, date, "a")

def add_signature():
    try:
        with open("signature.txt") as f:
            main_text.insert(tk.INSERT, "\n" + f.read(), "a")
    except IOError:
        MESSAGE = SIGNATURE_TXT_NOT_FOUND_MESSAGE
        pop_up.showerror("\"signature.txt\" not found.", MESSAGE)

root = tk.Tk()

menubar = tk.Menu(root)
menubar.add_command(label="Open", command=_open)
menubar.add_command(label="Save", command=save)
menubar.add_command(label="Add signature", command=add_signature)
menubar.add_command(label="Add date", command=add_date)


root.config(menu=menubar)

top = tk.Frame(root)
temp = tk.Label(root, text="Title:")
temp.pack(in_=top, side=tk.LEFT)

file_title = tk.Entry(root)
file_title.pack(in_=top, side=tk.RIGHT)

top.pack()

main_text = tk.Text(root)
main_text.pack()

tk.mainloop()


As I am talking about tools, it is probably worth mentionning that you'll find various other tools to check your code.

pylint finds :

************* Module txt
C: 13, 0: Line too long (105/80) (line-too-long)
C: 14, 0: Line too long (105/80) (line-too-long)
C: 15, 0: Line too long (141/80) (line-too-long)
C: 17, 0: Line too long (138/80) (line-too-long)
C:  1, 0: Missing module docstring (missing-docstring)
W:  3, 0: No exception type(s) specified (bare-except)
F:  4, 4: Unable to import 'Tkinter' (import-error)
W:  8, 0: No exception type(s) specified (bare-except)
F:  7, 4: Unable to import 'tkMessageBox' (import-error)
E:  9, 4: No name 'tkMessageBox' in module 'tkinter' (no-name-in-module)
F:  9, 4: Unable to import 'tkinter.tkMessageBox' (import-error)
C: 20, 0: Missing function docstring (missing-docstring)
C: 29,31: Invalid variable name "f" (invalid-name)
C: 36, 0: Missing function docstring (missing-docstring)
C: 44,33: Invalid variable name "f" (invalid-name)
C: 52, 0: Missing function docstring (missing-docstring)
C: 61, 0: Missing function docstring (missing-docstring)
C: 63,38: Invalid variable name "f" (invalid-name)
C: 66, 8: Invalid variable name "MESSAGE" (invalid-name)
C: 69, 0: Invalid constant name "root" (invalid-name)
C: 71, 0: Invalid constant name "menubar" (invalid-name)
C: 80, 0: Invalid constant name "top" (invalid-name)
C: 81, 0: Invalid constant name "temp" (invalid-name)
C: 84, 0: Invalid constant name "file_title" (invalid-name)
C: 89, 0: Invalid constant name "main_text" (invalid-name)


but there is also pyflakes, pychecker, etc.

Logic around file name

The following logic :

if not ".txt" in file_title.get():
    filename = file_title.get() + ".txt"


seems wrong on many different levels :


It does not follow PEP8 and should be if ".txt" not in file_title.get()
You have the same hardcoded string in multiple places : you are pretty much asking to be biten at some point if you want to change this. What about TXT_EXTENSION = '.txt'.
You are calling file_title.get() for no good reason
You do not define filename if the condition in the if is evaluated to false. Maybe you meant :

TXT_EXTENSION = '.txt'
filename = file_title.get()
if TXT_EXTENSION not in filename:
    filename = filename + TXT_EXTENSION
You are changing "toto.tutu" to "toto.tutu.txt" but "toto.txt.tutu" does not change. If what you are interested in is extension, you should probably be using endswith.


You have the same logic in multiple places : you should probably move it in a function of its own :


def add_txt_extension_if_needed(filename):
    if not filename.endswith(TXT_EXTENSION):
        return filename + TXT_EXTENSION
    return filename


Date format

You can probably find in the documentation how to format a date as a string.

I have to go and I don't know tkinter but I guess other people will have additional relevant comments.