xref: /third_party/python/Doc/library/idle.rst

.. _idle:

IDLE
====

.. moduleauthor:: Guido van Rossum <guido@python.org>

**Source code:** :source:`Lib/idlelib/`

.. index::
   single: IDLE
   single: Python Editor
   single: Integrated Development Environment

--------------

IDLE is Python's Integrated Development and Learning Environment.

IDLE has the following features:

* coded in 100% pure Python, using the :mod:`tkinter` GUI toolkit

* 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

Menus
-----

IDLE has two main window types, the Shell window and the Editor window.  It is
possible to have multiple editor windows simultaneously.  On Windows and
Linux, each has its own top menu.  Each menu documented below indicates
which window type it is associated with.

Output windows, such as used for Edit => Find in Files, are a subtype of editor
window.  They currently have the same top menu but a different
default title and context menu.

On macOS, there is one application menu.  It dynamically changes according
to the window currently selected.  It has an IDLE menu, and some entries
described below are moved around to conform to Apple guidelines.

File menu (Shell and Editor)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

New File
   Create a new file editing window.

Open...
   Open an existing file with an Open dialog.

Recent Files
   Open a list of recent files.  Click one to open it.

Open Module...
   Open an existing module (searches sys.path).

.. index::
   single: Class browser
   single: Path browser

Class Browser
   Show functions, classes, and methods in the current Editor file in a
   tree structure.  In the shell, open a module first.

Path Browser
   Show sys.path directories, modules, functions, classes and methods in a
   tree structure.

Save
   Save the current window to the associated file, if there is one.  Windows
   that have been changed since being opened or last saved have a \* before
   and after the window title.  If there is no associated file,
   do Save As instead.

Save As...
   Save the current window with a Save As dialog.  The file saved becomes the
   new associated file for the window.

Save Copy As...
   Save the current window to different file without changing the associated
   file.

Print Window
   Print the current window to the default printer.

Close
   Close the current window (ask to save if unsaved).

Exit
   Close all windows and quit IDLE (ask to save unsaved windows).

Edit menu (Shell and Editor)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Undo
   Undo the last change to the current window.  A maximum of 1000 changes may
   be undone.

Redo
   Redo the last undone change to the current window.

Cut
   Copy selection into the system-wide clipboard; then delete the selection.

Copy
   Copy selection into the system-wide clipboard.

Paste
   Insert contents of the system-wide clipboard into the current window.

The clipboard functions are also available in context menus.

Select All
   Select the entire contents of the current window.

Find...
   Open a search dialog with many options

Find Again
   Repeat the last search, if there is one.

Find Selection
   Search for the currently selected string, if there is one.

Find in Files...
   Open a file search dialog.  Put results in a new output window.

Replace...
   Open a search-and-replace dialog.

Go to Line
   Move the cursor to the beginning of the line requested and make that
   line visible.  A request past the end of the file goes to the end.
   Clear any selection and update the line and column status.

Show Completions
   Open a scrollable list allowing selection of existing names. See
   :ref:`Completions <completions>` in the Editing and navigation section below.

Expand Word
   Expand a prefix you have typed to match a full word in the same window;
   repeat to get a different expansion.

Show call tip
   After an unclosed parenthesis for a function, open a small window with
   function parameter hints.  See :ref:`Calltips <calltips>` in the
   Editing and navigation section below.

Show surrounding parens
   Highlight the surrounding parenthesis.

.. _format-menu:

Format menu (Editor window only)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Indent Region
   Shift selected lines right by the indent width (default 4 spaces).

Dedent Region
   Shift selected lines left by the indent width (default 4 spaces).

Comment Out Region
   Insert ## in front of selected lines.

Uncomment Region
   Remove leading # or ## from selected lines.

Tabify Region
   Turn *leading* stretches of spaces into tabs. (Note: We recommend using
   4 space blocks to indent Python code.)

Untabify Region
   Turn *all* tabs into the correct number of spaces.

Toggle Tabs
   Open a dialog to switch between indenting with spaces and tabs.

New Indent Width
   Open a dialog to change indent width. The accepted default by the Python
   community is 4 spaces.

Format Paragraph
   Reformat the current blank-line-delimited paragraph in comment block or
   multiline string or selected line in a string.  All lines in the
   paragraph will be formatted to less than N columns, where N defaults to 72.

Strip trailing whitespace
   Remove trailing space and other whitespace characters after the last
   non-whitespace character of a line by applying str.rstrip to each line,
   including lines within multiline strings.  Except for Shell windows,
   remove extra newlines at the end of the file.

.. index::
   single: Run script

Run menu (Editor window only)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. _run-module:

Run Module
   Do :ref:`Check Module <check-module>`.  If no error, restart the shell to clean the
   environment, then execute the module.  Output is displayed in the Shell
   window.  Note that output requires use of ``print`` or ``write``.
   When execution is complete, the Shell retains focus and displays a prompt.
   At this point, one may interactively explore the result of execution.
   This is similar to executing a file with ``python -i file`` at a command
   line.

.. _run-custom:

Run... Customized
   Same as :ref:`Run Module <run-module>`, but run the module with customized
   settings.  *Command Line Arguments* extend :data:`sys.argv` as if passed
   on a command line. The module can be run in the Shell without restarting.

.. _check-module:

Check Module
   Check the syntax of the module currently open in the Editor window. If the
   module has not been saved IDLE will either prompt the user to save or
   autosave, as selected in the General tab of the Idle Settings dialog.  If
   there is a syntax error, the approximate location is indicated in the
   Editor window.

.. _python-shell:

Python Shell
   Open or wake up the Python Shell window.


Shell menu (Shell window only)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

View Last Restart
  Scroll the shell window to the last Shell restart.

Restart Shell
  Restart the shell to clean the environment and reset display and exception handling.

Previous History
  Cycle through earlier commands in history which match the current entry.

Next History
  Cycle through later commands in history which match the current entry.

Interrupt Execution
  Stop a running program.

Debug menu (Shell window only)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Go to File/Line
   Look on the current line. with the cursor, and the line above for a filename
   and line number.  If found, open the file if not already open, and show the
   line.  Use this to view source lines referenced in an exception traceback
   and lines found by Find in Files. Also available in the context menu of
   the Shell window and Output windows.

.. index::
   single: debugger
   single: stack viewer

Debugger (toggle)
   When activated, code entered in the Shell or run from an Editor will run
   under the debugger.  In the Editor, breakpoints can be set with the context
   menu.  This feature is still incomplete and somewhat experimental.

Stack Viewer
   Show the stack traceback of the last exception in a tree widget, with
   access to locals and globals.

Auto-open Stack Viewer
   Toggle automatically opening the stack viewer on an unhandled exception.

Options menu (Shell and Editor)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Configure IDLE
   Open a configuration dialog and change preferences for the following:
   fonts, indentation, keybindings, text color themes, startup windows and
   size, additional help sources, and extensions.  On macOS, open the
   configuration dialog by selecting Preferences in the application
   menu. For more details, see
   :ref:`Setting preferences <preferences>` under Help and preferences.

Most configuration options apply to all windows or all future windows.
The option items below only apply to the active window.

Show/Hide Code Context (Editor Window only)
   Open a pane at the top of the edit window which shows the block context
   of the code which has scrolled above the top of the window.  See
   :ref:`Code Context <code-context>` in the Editing and Navigation section
   below.

Show/Hide Line Numbers (Editor Window only)
   Open a column to the left of the edit window which shows the number
   of each line of text.  The default is off, which may be changed in the
   preferences (see :ref:`Setting preferences <preferences>`).

Zoom/Restore Height
   Toggles the window between normal size and maximum height. The initial size
   defaults to 40 lines by 80 chars unless changed on the General tab of the
   Configure IDLE dialog.  The maximum height for a screen is determined by
   momentarily maximizing a window the first time one is zoomed on the screen.
   Changing screen settings may invalidate the saved height.  This toggle has
   no effect when a window is maximized.

Window menu (Shell and Editor)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Lists the names of all open windows; select one to bring it to the foreground
(deiconifying it if necessary).

Help menu (Shell and Editor)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

About IDLE
   Display version, copyright, license, credits, and more.

IDLE Help
   Display this IDLE document, detailing the menu options, basic editing and
   navigation, and other tips.

Python Docs
   Access local Python documentation, if installed, or start a web browser
   and open docs.python.org showing the latest Python documentation.

Turtle Demo
   Run the turtledemo module with example Python code and turtle drawings.

Additional help sources may be added here with the Configure IDLE dialog under
the General tab. See the :ref:`Help sources <help-sources>` subsection below
for more on Help menu choices.

.. index::
   single: Cut
   single: Copy
   single: Paste
   single: Set Breakpoint
   single: Clear Breakpoint
   single: breakpoints

Context Menus
^^^^^^^^^^^^^^^^^^^^^^^^^^

Open a context menu by right-clicking in a window (Control-click on macOS).
Context menus have the standard clipboard functions also on the Edit menu.

Cut
   Copy selection into the system-wide clipboard; then delete the selection.

Copy
   Copy selection into the system-wide clipboard.

Paste
   Insert contents of the system-wide clipboard into the current window.

Editor windows also have breakpoint functions.  Lines with a breakpoint set are
specially marked.  Breakpoints only have an effect when running under the
debugger.  Breakpoints for a file are saved in the user's ``.idlerc``
directory.

Set Breakpoint
   Set a breakpoint on the current line.

Clear Breakpoint
   Clear the breakpoint on that line.

Shell and Output windows also have the following.

Go to file/line
   Same as in Debug menu.

The Shell window also has an output squeezing facility explained in the *Python
Shell window* subsection below.

Squeeze
   If the cursor is over an output line, squeeze all the output between
   the code above and the prompt below down to a 'Squeezed text' label.


.. _editing-and-navigation:

Editing and navigation
----------------------

Editor windows
^^^^^^^^^^^^^^

IDLE may open editor windows when it starts, depending on settings
and how you start IDLE.  Thereafter, use the File menu.  There can be only
one open editor window for a given file.

The title bar contains the name of the file, the full path, and the version
of Python and IDLE running the window.  The status bar contains the line
number ('Ln') and column number ('Col').  Line numbers start with 1;
column numbers with 0.

IDLE assumes that files with a known .py* extension contain Python code
and that other files do not.  Run Python code with the Run menu.

Key bindings
^^^^^^^^^^^^

In this section, 'C' refers to the :kbd:`Control` key on Windows and Unix and
the :kbd:`Command` key on macOS.

* :kbd:`Backspace` deletes to the left; :kbd:`Del` deletes to the right

* :kbd:`C-Backspace` delete word left; :kbd:`C-Del` delete word to the right

* Arrow keys and :kbd:`Page Up`/:kbd:`Page Down` to move around

* :kbd:`C-LeftArrow` and :kbd:`C-RightArrow` moves by words

* :kbd:`Home`/:kbd:`End` go to begin/end of line

* :kbd:`C-Home`/:kbd:`C-End` go to begin/end of file

* Some useful Emacs bindings are inherited from Tcl/Tk:

   * :kbd:`C-a` beginning of line

   * :kbd:`C-e` end of line

   * :kbd:`C-k` kill line (but doesn't put it in clipboard)

   * :kbd:`C-l` center window around the insertion point

   * :kbd:`C-b` go backward one character without deleting (usually you can
     also use the cursor key for this)

   * :kbd:`C-f` go forward one character without deleting (usually you can
     also use the cursor key for this)

   * :kbd:`C-p` go up one line (usually you can also use the cursor key for
     this)

   * :kbd:`C-d` delete next character

Standard keybindings (like :kbd:`C-c` to copy and :kbd:`C-v` to paste)
may work.  Keybindings are selected in the Configure IDLE dialog.

Automatic indentation
^^^^^^^^^^^^^^^^^^^^^

After a block-opening statement, the next line is indented by 4 spaces (in the
Python Shell window by one tab).  After certain keywords (break, return etc.)
the next line is dedented.  In leading indentation, :kbd:`Backspace` deletes up
to 4 spaces if they are there. :kbd:`Tab` inserts spaces (in the Python
Shell window one tab), number depends on Indent width. Currently, tabs
are restricted to four spaces due to Tcl/Tk limitations.

See also the indent/dedent region commands on the
:ref:`Format menu <format-menu>`.

.. _completions:

Completions
^^^^^^^^^^^

Completions are supplied, when requested and available, for module
names, attributes of classes or functions, or filenames.  Each request
method displays a completion box with existing names.  (See tab
completions below for an exception.) For any box, change the name
being completed and the item highlighted in the box by
typing and deleting characters; by hitting :kbd:`Up`, :kbd:`Down`,
:kbd:`PageUp`, :kbd:`PageDown`, :kbd:`Home`, and :kbd:`End` keys;
and by a single click within the box.  Close the box with :kbd:`Escape`,
:kbd:`Enter`, and double :kbd:`Tab` keys or clicks outside the box.
A double click within the box selects and closes.

One way to open a box is to type a key character and wait for a
predefined interval.  This defaults to 2 seconds; customize it
in the settings dialog.  (To prevent auto popups, set the delay to a
large number of milliseconds, such as 100000000.) For imported module
names or class or function attributes, type '.'.
For filenames in the root directory, type :data:`os.sep` or
:data:`os.altsep` immediately after an opening quote.  (On Windows,
one can specify a drive first.)  Move into subdirectories by typing a
directory name and a separator.

Instead of waiting, or after a box is closed, open a completion box
immediately with Show Completions on the Edit menu.  The default hot
key is :kbd:`C-space`.  If one types a prefix for the desired name
before opening the box, the first match or near miss is made visible.
The result is the same as if one enters a prefix
after the box is displayed.  Show Completions after a quote completes
filenames in the current directory instead of a root directory.

Hitting :kbd:`Tab` after a prefix usually has the same effect as Show
Completions.  (With no prefix, it indents.)  However, if there is only
one match to the prefix, that match is immediately added to the editor
text without opening a box.

Invoking 'Show Completions', or hitting :kbd:`Tab` after a prefix,
outside of a string and without a preceding '.' opens a box with
keywords, builtin names, and available module-level names.

When editing code in an editor (as oppose to Shell), increase the
available module-level names by running your code
and not restarting the Shell thereafter.  This is especially useful
after adding imports at the top of a file.  This also increases
possible attribute completions.

Completion boxes initially exclude names beginning with '_' or, for
modules, not included in '__all__'.  The hidden names can be accessed
by typing '_' after '.', either before or after the box is opened.

.. _calltips:

Calltips
^^^^^^^^

A calltip is shown automatically when one types :kbd:`(` after the name
of an *accessible* function.  A function name expression may include
dots and subscripts.  A calltip remains until it is clicked, the cursor
is moved out of the argument area, or :kbd:`)` is typed.  Whenever the
cursor is in the argument part of a definition, select Edit and "Show
Call Tip" on the menu or enter its shortcut to display a calltip.

The calltip consists of the function's signature and docstring up to
the latter's first blank line or the fifth non-blank line.  (Some builtin
functions lack an accessible signature.)  A '/' or '*' in the signature
indicates that the preceding or following arguments are passed by
position or name (keyword) only.  Details are subject to change.

In Shell, the accessible functions depends on what modules have been
imported into the user process, including those imported by Idle itself,
and which definitions have been run, all since the last restart.

For example, restart the Shell and enter ``itertools.count(``.  A calltip
appears because Idle imports itertools into the user process for its own
use.  (This could change.)  Enter ``turtle.write(`` and nothing appears.
Idle does not itself import turtle.  The menu entry and shortcut also do
nothing.  Enter ``import turtle``.  Thereafter, ``turtle.write(``
will display a calltip.

In an editor, import statements have no effect until one runs the file.
One might want to run a file after writing import statements, after
adding function definitions, or after opening an existing file.

.. _code-context:

Code Context
^^^^^^^^^^^^

Within an editor window containing Python code, code context can be toggled
in order to show or hide a pane at the top of the window.  When shown, this
pane freezes the opening lines for block code, such as those beginning with
``class``, ``def``, or ``if`` keywords, that would have otherwise scrolled
out of view.  The size of the pane will be expanded and contracted as needed
to show the all current levels of context, up to the maximum number of
lines defined in the Configure IDLE dialog (which defaults to 15).  If there
are no current context lines and the feature is toggled on, a single blank
line will display.  Clicking on a line in the context pane will move that
line to the top of the editor.

The text and background colors for the context pane can be configured under
the Highlights tab in the Configure IDLE dialog.

Python Shell window
^^^^^^^^^^^^^^^^^^^

With IDLE's Shell, one enters, edits, and recalls complete statements.
Most consoles and terminals only work with a single physical line at a time.

When one pastes code into Shell, it is not compiled and possibly executed
until one hits :kbd:`Return`.  One may edit pasted code first.
If one pastes more that one statement into Shell, the result will be a
:exc:`SyntaxError` when multiple statements are compiled as if they were one.

The editing features described in previous subsections work when entering
code interactively.  IDLE's Shell window also responds to the following keys.

* :kbd:`C-c` interrupts executing command

* :kbd:`C-d` sends end-of-file; closes window if typed at a ``>>>`` prompt

* :kbd:`Alt-/` (Expand word) is also useful to reduce typing

  Command history

  * :kbd:`Alt-p` retrieves previous command matching what you have typed. On
    macOS use :kbd:`C-p`.

  * :kbd:`Alt-n` retrieves next. On macOS use :kbd:`C-n`.

  * :kbd:`Return` while on any previous command retrieves that command

Text colors
^^^^^^^^^^^

Idle defaults to black on white text, but colors text with special meanings.
For the shell, these are shell output, shell error, user output, and
user error.  For Python code, at the shell prompt or in an editor, these are
keywords, builtin class and function names, names following ``class`` and
``def``, strings, and comments. For any text window, these are the cursor (when
present), found text (when possible), and selected text.

IDLE also highlights the :ref:`soft keywords <soft-keywords>` :keyword:`match`,
:keyword:`case <match>`, and :keyword:`_ <wildcard-patterns>` in
pattern-matching statements. However, this highlighting is not perfect and
will be incorrect in some rare cases, including some ``_``-s in ``case``
patterns.

Text coloring is done in the background, so uncolorized text is occasionally
visible.  To change the color scheme, use the Configure IDLE dialog
Highlighting tab.  The marking of debugger breakpoint lines in the editor and
text in popups and dialogs is not user-configurable.


Startup and code execution
--------------------------

Upon startup with the ``-s`` option, IDLE will execute the file referenced by
the environment variables :envvar:`IDLESTARTUP` or :envvar:`PYTHONSTARTUP`.
IDLE first checks for ``IDLESTARTUP``; if ``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.

In addition, ``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
^^^^^^^^^^^^^^^^^^

.. code-block:: none

   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:

* If ``-``, ``-c``, or ``r`` is used, all arguments are placed in
  ``sys.argv[1:...]`` and ``sys.argv[0]`` is set to ``''``, ``'-c'``,
  or ``'-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.argv`` reflects the arguments passed to IDLE itself.

Startup failure
^^^^^^^^^^^^^^^

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
with ``** Invalid host:``.
The valid value is ``127.0.0.1 (idlelib.rpc.LOCALHOST)``.
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 ``8.6.11`` (see
``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,
and ``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 ``sys.stdin``, ``sys.stdout``, and ``sys.stderr``
with objects that get input from and send output to the Shell window.
The original values stored in ``sys.__stdin__``, ``sys.__stdout__``, and
``sys.__stderr__`` are not touched, but may be ``None``.

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
and :meth:`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 ``input`` from
sys.stdin or ``print`` or ``write`` to sys.stdout or sys.stderr,
IDLE should be started in a command line window.  The secondary subprocess
will then be attached to that window for input and output.

If ``sys`` is reset by user code, such as with ``importlib.reload(sys)``,
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.getrecursionlimit`` and
``sys.setrecursionlimit`` to reduce the effect of the additional stack
frames.

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.stdout``
and ``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.
   >>> len(s)
   14
   >>> s  # Display repr(s)
   'a\tb\x07<\x02><\r>\x08c\nd'
   >>> print(s, end='')  # Display s as is.
   # Result varies by OS and font.  Try it.

The ``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.
Next enter ``b = tk.Button(root, text='button'); b.pack()``.  Again,
nothing visibly changes in standard Python until one enters ``root.update()``.

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
appear until ``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:: 3.4


Help and preferences
--------------------

.. _help-sources:

Help sources
^^^^^^^^^^^^

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.

.. _preferences:

Setting preferences
^^^^^^^^^^^^^^^^^^^

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 ``.idlerc``.

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.

Extensions
^^^^^^^^^^

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.