IDLE — Python editor and shell
******************************

**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


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.

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

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

Module 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. (If your file
   namager is set to hide extensions, the current extension will be
   omitted in the file name box. If the new filename has no ‘.’, ‘.py’
   and ‘.txt’ will be added for Python and text files, except that on
   macOS Aqua,’.py’ is added for all files.)

Save Copy As…
   Save the current window to different file without changing the
   associated file.  (See Save As note above about filename
   extensions.)

Print Window
   Print the current window to the default printer.

Close Window
   Close the current window (if an unsaved editor, ask to save; if an
   unsaved Shell, ask to quit execution).  Calling "exit()" or
   "close()" in the Shell window also closes Shell.  If this is the
   only window, also exit IDLE.

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

Select All
   Select the entire contents of 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.

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
   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 Calltips in the Editing and
   navigation section below.

Show Surrounding Parens
   Highlight the surrounding parenthesis.


Format menu (Editor window only)
--------------------------------

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.

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.

Strip Trailing Chitespace
   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.


Run menu (Editor window only)
-----------------------------

Run Module
   Do 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… Customized
   Same as Run Module, but run the module with customized settings.
   *Command Line Arguments* extend "sys.argv" as if passed on a
   command line. The module can be run in the Shell without
   restarting.

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
   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.

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 Setting 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 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 Setting 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 Help sources subsection below
for more on Help menu choices.


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
======================


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

The IDLE insertion cursor is a thin vertical bar between character
positions.  When characters are entered, the insertion cursor and
everything to its right moves right one character and the new
character is entered in the new space.

Several non-character keys move the cursor and possibly delete
characters.  Deletion does not puts text on the clipboard, but IDLE
has an undo list.  Wherever this doc discusses keys, ‘C’ refers to the
"Control" key on Windows and Unix and the "Command" key on macOS.
(And all such discussions assume that the keys have not been re-bound
to something else.)

* Arrow keys move the cursor one character or line.

* "C"-"LeftArrow" and "C"-"RightArrow" moves left or right one word.

* "Home" and "End" go to the beginning or end of the line.

* "Page Up" and "Page Down" go up or down one screen.

* "C"-"Home" and "C"-"End" go to beginning or end of the file.

* "Backspace" and "Del" (or "C"-"d") delete the previous or next
  character.

* "C"-"Backspace" and "C"-"Del" delete one word left or right.

* "C"-"k" deletes (‘kills’) everything to the right.

Standard keybindings (like "C"-"c" to copy and "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, "Backspace" deletes up to 4 spaces if they are there.
"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 Format menu.


Search and Replace
------------------

Any selection becomes a search target.  However, only selections
within a line work because searches are only performed within lines
with the terminal newline removed.  If "[x] Regular expression" is
checked, the target is interpreted according to the Python re module.


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 "Up", "Down", "PageUp", "PageDown",
"Home", and "End" keys; and by a single click within the box.  Close
the box with "Escape", "Enter", and double "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 "os.sep" or "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 "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 "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 "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
--------

A calltip is shown automatically when one types "(" 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 ")" 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
------------

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.


Shell window
------------

In IDLE’s Shell, enter, edit, and recall complete statements. (Most
consoles and terminals only work with a single physical line at a
time).

Submit a single-line statement for execution by hitting "Return" with
the cursor anywhere on the line.  If a line is extended with Backslash
("\"), the cursor must be on the last physical line. Submit a multi-
line compound statement by entering a blank line after the statement.

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

Lines containing "RESTART" mean that the user execution process has
been re-started.  This occurs when the user execution process has
crashed, when one requests a restart on the Shell menu, or when one
runs code in an editor window.

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

* "C"-"c" attempts to interrupt statement execution (but may fail).

* "C"-"d" closes Shell if typed at a ">>>" prompt.

* "Alt"-"p" and "Alt"-"n" ("C"-"p" and "C"-"n" on macOS) retrieve to
  the current prompt the previous or next previously entered statement
  that matches anything already typed.

* "Return" while the cursor is on any previous statement appends the
  latter to anything already typed at the prompt.


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 soft keywords "match", "case", and "_" 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 "IDLESTARTUP" or
"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
------------------

IDLE can be invoked from the command line with various options. The
general syntax is:

   python -m idlelib [options] [file ...]

The following options are available:

-c <command>

   Run the specified Python command in the shell window. For example,
   pass "-c "print('Hello, World!')"". On Windows, the outer quotes
   must be double quotes as shown.

-d

   Enable the debugger and open the shell window.

-e

   Open an editor window.

-h

   Print a help message with legal combinations of options and exit.

-i

   Open a shell window.

-r <file>

   Run the specified file in the shell window.

-s

   Run the startup file (as defined by the environment variables
   "IDLESTARTUP" or "PYTHONSTARTUP") before opening the shell window.

-t <title>

   Set the title of the shell window.

-

   Read and execute standard input in the shell window. This option
   must be the last one before any arguments.

If arguments are provided:

* If "-", "-c", or "-r" is used, all arguments are placed in
  "sys.argv[1:]", and "sys.argv[0]" is set to "''", "'-c'", or "'-r'"
  respectively. No editor window is opened, even if that is the
  default set in the *Options* dialog.

* Otherwise, arguments are treated as files to be 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 "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.  (On Windows, use "python"
or "py" rather than "pythonw" or "pyw".) 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 since version 3.4.


Help and Preferences
====================


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.


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.


idlelib — implementation of IDLE application
============================================

**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**).
