Source code: Lib/idlelib/
IDLE is Python’s Integrated Development and Learning Environment.
IDLE has the following features:
cross-platform: works mostly the same on Windows, Unix, and macOS
Python shell window (interactive interpreter) with colorizing of code input, output, and error messages
multi-window text editor with multiple undo, Python colorizing, smart indent, call tips, auto completion, and other features
search within any window, replace within editor windows, and search through multiple files (grep)
debugger with persistent breakpoints, stepping, and viewing of global and local namespaces
configuration, browsers, and other dialogs
Startup and Code Execution¶
Upon startup with the
-s option, IDLE will execute the file referenced by
the environment variables
IDLE first checks for
IDLESTARTUP is present the file
referenced is run. If
IDLESTARTUP is not present, IDLE checks for
PYTHONSTARTUP. Files referenced by these environment variables are
convenient places to store functions that are used frequently from the IDLE
shell, or for executing import statements to import common modules.
Tk also loads a startup file if it is present. Note that the
Tk file is loaded unconditionally. This additional file is
.Idle.py and is
looked for in the user’s home directory. Statements in this file will be
executed in the Tk namespace, so this file is not useful for importing
functions to be used from IDLE’s Python shell.
Command line usage¶
idle.py [-c command] [-d] [-e] [-h] [-i] [-r file] [-s] [-t title] [-] [arg] ...
-c command run command in the shell window
-d enable debugger and open shell window
-e open editor window
-h print help message with legal combinations and exit
-i open shell window
-r file run file in shell window
-s run $IDLESTARTUP or $PYTHONSTARTUP first, in shell window
-t title set title of shell window
- run stdin in shell (- must be last option before args)
If there are arguments:
ris used, all arguments are placed in
sys.argvis set to
'-r'. No editor window is opened, even if that is the default set in the Options dialog.
Otherwise, arguments are files opened for editing and
sys.argvreflects the arguments passed to IDLE itself.
IDLE uses a socket to communicate between the IDLE GUI process and the user
code execution process. A connection must be established whenever the Shell
starts or restarts. (The latter is indicated by a divider line that says
‘RESTART’). If the user process fails to connect to the GUI process, it
usually displays a
Tk error box with a ‘cannot connect’ message
that directs the user here. It then exits.
One specific connection failure on Unix systems results from
misconfigured masquerading rules somewhere in a system’s network setup.
When IDLE is started from a terminal, one will see a message starting
** Invalid host:.
The valid value is
One can diagnose with
tcpconnect -irv 127.0.0.1 6543 in one
terminal window and
tcplisten <same args> in another.
A common cause of failure is a user-written file with the same name as a standard library module, such as random.py and tkinter.py. When such a file is located in the same directory as a file that is about to be run, IDLE cannot import the stdlib file. The current fix is to rename the user file.
Though less common than in the past, an antivirus or firewall program may stop the connection. If the program cannot be taught to allow the connection, then it must be turned off for IDLE to work. It is safe to allow this internal connection because no data is visible on external ports. A similar problem is a network mis-configuration that blocks connections.
Python installation issues occasionally stop IDLE: multiple versions can clash, or a single installation might need admin access. If one undo the clash, or cannot or does not want to run as admin, it might be easiest to completely remove Python and start over.
A zombie pythonw.exe process could be a problem. On Windows, use Task Manager to check for one and stop it if there is. Sometimes a restart initiated by a program crash or Keyboard Interrupt (control-C) may fail to connect. Dismissing the error box or using Restart Shell on the Shell menu may fix a temporary problem.
When IDLE first starts, it attempts to read user configuration files in
~/.idlerc/ (~ is one’s home directory). If there is a problem, an error
message should be displayed. Leaving aside random disk glitches, this can
be prevented by never editing the files by hand. Instead, use the
configuration dialog, under Options. Once there is an error in a user
configuration file, the best solution may be to delete it and start over
with the settings dialog.
If IDLE quits with no message, and it was not started from a console, try
starting it from a console or terminal (
python -m idlelib) and see if
this results in an error message.
On Unix-based systems with tcl/tk older than
About IDLE) certain characters of certain fonts can cause
a tk failure with a message to the terminal. This can happen either
if one starts IDLE to edit a file with such a character or later
when entering such a character. If one cannot upgrade tcl/tk,
then re-configure IDLE to use a font that works better.
Running user code¶
With rare exceptions, the result of executing Python code with IDLE is
intended to be the same as executing the same code by the default method,
directly with Python in a text-mode system console or terminal window.
However, the different interface and operation occasionally affect
visible results. For instance,
sys.modules starts with more entries,
threading.active_count() returns 2 instead of 1.
By default, IDLE runs user code in a separate OS process rather than in
the user interface process that runs the shell and editor. In the execution
process, it replaces
with objects that get input from and send output to the Shell window.
The original values stored in
sys.__stderr__ are not touched, but may be
Sending print output from one process to a text widget in another is
slower than printing to a system terminal in the same process.
This has the most effect when printing multiple arguments, as the string
for each argument, each separator, the newline are sent separately.
For development, this is usually not a problem, but if one wants to
print faster in IDLE, format and join together everything one wants
displayed together and then print a single string. Both format strings
str.join() can help combine fields and lines.
IDLE’s standard stream replacements are not inherited by subprocesses
created in the execution process, whether directly by user code or by
modules such as multiprocessing. If such subprocess use
write to sys.stdout or sys.stderr,
IDLE should be started in a command line window. (On Windows,
py rather than
The secondary subprocess
will then be attached to that window for input and output.
sys is reset by user code, such as with
IDLE’s changes are lost and input from the keyboard and output to the screen
will not work correctly.
When Shell has the focus, it controls the keyboard and screen. This is normally transparent, but functions that directly access the keyboard and screen will not work. These include system-specific functions that determine whether a key has been pressed and if so, which.
The IDLE code running in the execution process adds frames to the call stack
that would not be there otherwise. IDLE wraps
sys.setrecursionlimit to reduce the effect of the additional stack
When user code raises SystemExit either directly or by calling sys.exit, IDLE returns to a Shell prompt instead of exiting.
User output in Shell¶
When a program outputs text, the result is determined by the
corresponding output device. When IDLE executes user code,
sys.stderr are connected to the display area of IDLE’s Shell. Some of
its features are inherited from the underlying Tk Text widget. Others
are programmed additions. Where it matters, Shell is designed for development
rather than production runs.
For instance, Shell never throws away output. A program that sends unlimited output to Shell will eventually fill memory, resulting in a memory error. In contrast, some system text windows only keep the last n lines of output. A Windows console, for instance, keeps a user-settable 1 to 9999 lines, with 300 the default.
A Tk Text widget, and hence IDLE’s Shell, displays characters (codepoints) in the BMP (Basic Multilingual Plane) subset of Unicode. Which characters are displayed with a proper glyph and which with a replacement box depends on the operating system and installed fonts. Tab characters cause the following text to begin after the next tab stop. (They occur every 8 ‘characters’). Newline characters cause following text to appear on a new line. Other control characters are ignored or displayed as a space, box, or something else, depending on the operating system and font. (Moving the text cursor through such output with arrow keys may exhibit some surprising spacing behavior.)
>>> s = 'a\tb\a<\x02><\r>\bc\nd' # Enter 22 chars.
>>> s # Display repr(s)
>>> print(s, end='') # Display s as is.
# Result varies by OS and font. Try it.
repr function is used for interactive echo of expression
values. It returns an altered version of the input string in which
control codes, some BMP codepoints, and all non-BMP codepoints are
replaced with escape codes. As demonstrated above, it allows one to
identify the characters in a string, regardless of how they are displayed.
Normal and error output are generally kept separate (on separate lines) from code input and each other. They each get different highlight colors.
For SyntaxError tracebacks, the normal ‘^’ marking where the error was detected is replaced by coloring the text with an error highlight. When code run from a file causes other exceptions, one may right click on a traceback line to jump to the corresponding line in an IDLE editor. The file will be opened if necessary.
Shell has a special facility for squeezing output lines down to a ‘Squeezed text’ label. This is done automatically for output over N lines (N = 50 by default). N can be changed in the PyShell section of the General page of the Settings dialog. Output with fewer lines can be squeezed by right clicking on the output. This can be useful lines long enough to slow down scrolling.
Squeezed output is expanded in place by double-clicking the label. It can also be sent to the clipboard or a separate view window by right-clicking the label.
Developing tkinter applications¶
IDLE is intentionally different from standard Python in order to
facilitate development of tkinter programs. Enter
import tkinter as tk;
root = tk.Tk() in standard Python and nothing appears. Enter the same
in IDLE and a tk window appears. In standard Python, one must also enter
root.update() to see the window. IDLE does the equivalent in the
background, about 20 times a second, which is about every 50 milliseconds.
b = tk.Button(root, text='button'); b.pack(). Again,
nothing visibly changes in standard Python until one enters
Most tkinter programs run
root.mainloop(), which usually does not
return until the tk app is destroyed. If the program is run with
python -i or from an IDLE editor, a
>>> shell prompt does not
mainloop() returns, at which time there is nothing left
to interact with.
When running a tkinter program from an IDLE editor, one can comment out the mainloop call. One then gets a shell prompt immediately and can interact with the live application. One just has to remember to re-enable the mainloop call when running in standard Python.
Running without a subprocess¶
By default, IDLE executes user code in a separate subprocess via a socket, which uses the internal loopback interface. This connection is not externally visible and no data is sent to or received from the internet. If firewall software complains anyway, you can ignore it.
If the attempt to make the socket connection fails, Idle will notify you. Such failures are sometimes transient, but if persistent, the problem may be either a firewall blocking the connection or misconfiguration of a particular system. Until the problem is fixed, one can run Idle with the -n command line switch.
If IDLE is started with the -n command line switch it will run in a single process and will not create the subprocess which runs the RPC Python execution server. This can be useful if Python cannot create the subprocess or the RPC socket interface on your platform. However, in this mode user code is not isolated from IDLE itself. Also, the environment is not restarted when Run/Run Module (F5) is selected. If your code has been modified, you must reload() the affected modules and re-import any specific items (e.g. from foo import baz) if the changes are to take effect. For these reasons, it is preferable to run IDLE with the default subprocess if at all possible.
Deprecated since version 3.4.
Help and Preferences¶
Help menu entry “IDLE Help” displays a formatted html version of the IDLE chapter of the Library Reference. The result, in a read-only tkinter text window, is close to what one sees in a web browser. Navigate through the text with a mousewheel, the scrollbar, or up and down arrow keys held down. Or click the TOC (Table of Contents) button and select a section header in the opened box.
Help menu entry “Python Docs” opens the extensive sources of help,
including tutorials, available at
docs.python.org/x.y, where ‘x.y’
is the currently running Python version. If your system
has an off-line copy of the docs (this may be an installation option),
that will be opened instead.
Selected URLs can be added or removed from the help menu at any time using the General tab of the Configure IDLE dialog.
The font preferences, highlighting, keys, and general preferences can be
changed via Configure IDLE on the Option menu.
Non-default user settings are saved in a
.idlerc directory in the user’s
home directory. Problems caused by bad user configuration files are solved
by editing or deleting one or more of the files in
On the Font tab, see the text sample for the effect of font face and size on multiple characters in multiple languages. Edit the sample to add other characters of personal interest. Use the sample to select monospaced fonts. If particular characters have problems in Shell or an editor, add them to the top of the sample and try changing first size and then font.
On the Highlights and Keys tab, select a built-in or custom color theme and key set. To use a newer built-in color theme or key set with older IDLEs, save it as a new custom theme or key set and it well be accessible to older IDLEs.
IDLE on macOS¶
Under System Preferences: Dock, one can set “Prefer tabs when opening documents” to “Always”. This setting is not compatible with the tk/tkinter GUI framework used by IDLE, and it breaks a few IDLE features.
IDLE contains an extension facility. Preferences for extensions can be changed with the Extensions tab of the preferences dialog. See the beginning of config-extensions.def in the idlelib directory for further information. The only current default extension is zzdummy, an example also used for testing.
Source code: Lib/idlelib
The Lib/idlelib package implements the IDLE application. See the rest of this page for how to use IDLE.
The files in idlelib are described in idlelib/README.txt. Access it either in idlelib or click Help => About IDLE on the IDLE menu. This file also maps IDLE menu items to the code that implements the item. Except for files listed under ‘Startup’, the idlelib code is ‘private’ in sense that feature changes can be backported (see PEP 434).