用 Python 进行 Curses 编程¶

作者: A.M. Kuchling, Eric S. Raymond
发布版本: 2.03

摘要

This document describes how to write text-mode programs with Python 2.x, using the curses extension module to control the display.

curses 是什么？¶

The curses library supplies a terminal-independent screen-painting and keyboard-handling facility for text-based terminals; such terminals include VT100s, the Linux console, and the simulated terminal provided by X11 programs such as xterm and rxvt. Display terminals support various control codes to perform common operations such as moving the cursor, scrolling the screen, and erasing areas. Different terminals use widely differing codes, and often have their own minor quirks.

In a world of X displays, one might ask “why bother”? It’s true that character-cell display terminals are an obsolete technology, but there are niches in which being able to do fancy things with them are still valuable. One is on small-footprint or embedded Unixes that don’t carry an X server. Another is for tools like OS installers and kernel configurators that may have to run before X is available.

The curses library hides all the details of different terminals, and provides the programmer with an abstraction of a display, containing multiple non-overlapping windows. The contents of a window can be changed in various ways—adding text, erasing it, changing its appearance—and the curses library will automagically figure out what control codes need to be sent to the terminal to produce the right output.

curses 库最初是为BSD Unix 编写的。 AT＆T 的Unix 的后来的System V 版本增加了许多增强功能和新功能。如今BSD curses不再维护，被ncurses取代，ncurses是AT＆T接口的开源实现。如果您使用的是Linux 或FreeBSD 等开源Unix，则您的系统几乎肯定会使用ncurses。由于大多数当前的商业Unix版本都基于System V代码，因此这里描述的所有功能可能都可用。但是，某些专有Unix所带来的较早版本的curses可能无法支持所有功能。

No one has made a Windows port of the curses module. On a Windows platform, try the Console module written by Fredrik Lundh. The Console module provides cursor-addressable text output, plus full support for mouse and keyboard input, and is available from http://effbot.org/zone/console-index.htm.

Python 的 curses 模块¶

Thy Python module is a fairly simple wrapper over the C functions provided by curses; if you’re already familiar with curses programming in C, it’s really easy to transfer that knowledge to Python. The biggest difference is that the Python interface makes things simpler, by merging different C functions such as addstr(), mvaddstr(), mvwaddstr(), into a single addstr() method. You’ll see this covered in more detail later.

This HOWTO is simply an introduction to writing text-mode programs with curses and Python. It doesn’t attempt to be a complete guide to the curses API; for that, see the Python library guide’s section on ncurses, and the C manual pages for ncurses. It will, however, give you the basic ideas.

开始和结束curses应用程序¶

Before doing anything, curses must be initialized. This is done by calling the initscr() function, which will determine the terminal type, send any required setup codes to the terminal, and create various internal data structures. If successful, initscr() returns a window object representing the entire screen; this is usually called stdscr, after the name of the corresponding C variable.

import curses
stdscr = curses.initscr()

Usually curses applications turn off automatic echoing of keys to the screen, in order to be able to read keys and only display them under certain circumstances. This requires calling the noecho() function.

curses.noecho()

应用程序也会广泛地需要立即响应按键，而不需要按下回车键；这被称为 “cbreak” 模式，与通常的缓冲输入模式相对：

curses.cbreak()

终端通常会以多字节转义序列的形式返回特殊按键，比如光标键和导航键比如 Page Up 键和 Home 键。尽管你可以编写你的程序来应对这些序列，curses 能够代替你做到这件事，返回一个特殊值比如 curses.KEY_LEFT。为了让 curses 做这项工作，你需要启用 keypad 模式：

stdscr.keypad(1)

Terminating a curses application is much easier than starting one. You’ll need to call

curses.nocbreak(); stdscr.keypad(0); curses.echo()

to reverse the curses-friendly terminal settings. Then call the endwin() function to restore the terminal to its original operating mode.

curses.endwin()

调试一个 curses 应用程序时常会出现，一个应用程序还未能还原终端到原本的状态就意外退出了，这会搅乱你的终端。在 Python 中这常常会发生在你的代码中有 bug 并抛出了一个未捕获的异常。当你尝试输入时按键不会上屏，这使得使用终端变得困难。

In Python you can avoid these complications and make debugging much easier by importing the curses.wrapper() function. It takes a callable and does the initializations described above, also initializing colors if color support is present. It then runs your provided callable and finally deinitializes appropriately. The callable is called inside a try-catch clause which catches exceptions, performs curses deinitialization, and then passes the exception upwards. Thus, your terminal won’t be left in a funny state on exception.

Windows 和 Pad¶

Windows are the basic abstraction in curses. A window object represents a rectangular area of the screen, and supports various methods to display text, erase it, allow the user to input strings, and so forth.

The stdscr object returned by the initscr() function is a window object that covers the entire screen. Many programs may need only this single window, but you might wish to divide the screen into smaller windows, in order to redraw or clear them separately. The newwin() function creates a new window of a given size, returning the new window object.

begin_x = 20; begin_y = 7
height = 5; width = 40
win = curses.newwin(height, width, begin_y, begin_x)

A word about the coordinate system used in curses: coordinates are always passed in the order y,x, and the top-left corner of a window is coordinate (0,0). This breaks a common convention for handling coordinates, where the x coordinate usually comes first. This is an unfortunate difference from most other computer applications, but it’s been part of curses since it was first written, and it’s too late to change things now.

When you call a method to display or erase text, the effect doesn’t immediately show up on the display. This is because curses was originally written with slow 300-baud terminal connections in mind; with these terminals, minimizing the time required to redraw the screen is very important. This lets curses accumulate changes to the screen, and display them in the most efficient manner. For example, if your program displays some characters in a window, and then clears the window, there’s no need to send the original characters because they’d never be visible.

Accordingly, curses requires that you explicitly tell it to redraw windows, using the refresh() method of window objects. In practice, this doesn’t really complicate programming with curses much. Most programs go into a flurry of activity, and then pause waiting for a keypress or some other action on the part of the user. All you have to do is to be sure that the screen has been redrawn before pausing to wait for user input, by simply calling stdscr.refresh() or the refresh() method of some other relevant window.

A pad is a special case of a window; it can be larger than the actual display screen, and only a portion of it displayed at a time. Creating a pad simply requires the pad’s height and width, while refreshing a pad requires giving the coordinates of the on-screen area where a subsection of the pad will be displayed.

pad = curses.newpad(100, 100)
#  These loops fill the pad with letters; this is
# explained in the next section
for y in range(0, 100):
    for x in range(0, 100):
        try:
            pad.addch(y,x, ord('a') + (x*x+y*y) % 26)
        except curses.error:
            pass

#  Displays a section of the pad in the middle of the screen
pad.refresh(0,0, 5,5, 20,75)

The refresh() call displays a section of the pad in the rectangle extending from coordinate (5,5) to coordinate (20,75) on the screen; the upper left corner of the displayed section is coordinate (0,0) on the pad. Beyond that difference, pads are exactly like ordinary windows and support the same methods.

If you have multiple windows and pads on screen there is a more efficient way to go, which will prevent annoying screen flicker at refresh time. Use the noutrefresh() method of each window to update the data structure representing the desired state of the screen; then change the physical screen to match the desired state in one go with the function doupdate(). The normal refresh() method calls doupdate() as its last act.

显示文字¶

From a C programmer’s point of view, curses may sometimes look like a twisty maze of functions, all subtly different. For example, addstr() displays a string at the current cursor location in the stdscr window, while mvaddstr() moves to a given y,x coordinate first before displaying the string. waddstr() is just like addstr(), but allows specifying a window to use, instead of using stdscr by default. mvwaddstr() follows similarly.

Fortunately the Python interface hides all these details; stdscr is a window object like any other, and methods like addstr() accept multiple argument forms. Usually there are four different forms.

形式	描述
str 或 ch	在当前位置显示字符串 str 或字符 ch
str 或 ch, attr	在当前位置使用 attr 属性显示字符串 str 或字符 ch
y, x, str 或 ch	移动到窗口内的 y,x 位置，并显示 str 或 ch
y, x, str 或 ch, attr	移至窗口内的 y,x 位置，并使用 attr 属性显示 str 或 ch

Attributes allow displaying text in highlighted forms, such as in boldface, underline, reverse code, or in color. They’ll be explained in more detail in the next subsection.

The addstr() function takes a Python string as the value to be displayed, while the addch() functions take a character, which can be either a Python string of length 1 or an integer. If it’s a string, you’re limited to displaying characters between 0 and 255. SVr4 curses provides constants for extension characters; these constants are integers greater than 255. For example, ACS_PLMINUS is a +/- symbol, and ACS_ULCORNER is the upper left corner of a box (handy for drawing borders).

窗口会记住上次操作之后光标所在位置，所以如果你忽略 y,x 坐标，字符串和字符会出现在上次操作结束的位置。你也可以通过 move(y,x) 的方法来移动光标。因为一些终端始终会显示一个闪烁的光标，你可能会想要保证光标处于一些不会让人感到分心的位置。在看似随机的位置出现一个闪烁的光标会令人非常迷惑。

If your application doesn’t need a blinking cursor at all, you can call curs_set(0) to make it invisible. Equivalently, and for compatibility with older curses versions, there’s a leaveok(bool) function. When bool is true, the curses library will attempt to suppress the flashing cursor, and you won’t need to worry about leaving it in odd locations.

属性和颜色¶

Characters can be displayed in different ways. Status lines in a text-based application are commonly shown in reverse video; a text viewer may need to highlight certain words. curses supports this by allowing you to specify an attribute for each cell on the screen.

属性值是一个整数，它的每一个二进制位代表一个不同的属性。你可以尝试以多种不属性位组合来显示文本，但 curses 不保证所有的组合都是有效的，或者看上去有明显不同。这一点取决于用户终端的能力，所以最稳妥的方式是只采用最常见的有效属性，见下表。

属性	描述
`A_BLINK`	闪烁文字
`A_BOLD`	超亮或粗体文字
`A_DIM`	半明亮的文字
`A_REVERSE`	反向视频文本
`A_STANDOUT`	可用的最佳突出显示模式
`A_UNDERLINE`	带下划线的文字

所以，为了在屏幕顶部显示一个反相的状态行，你可以这么编写：

stdscr.addstr(0, 0, "Current mode: Typing mode",
              curses.A_REVERSE)
stdscr.refresh()

curses 库还支持在提供了颜色功能的终端上显示颜色的功能。最常见的提供颜色的终端很可能是 Linux 控制台，采用了 xterms 配色方案。

To use color, you must call the start_color() function soon after calling initscr(), to initialize the default color set (the curses.wrapper.wrapper() function does this automatically). Once that’s done, the has_colors() function returns TRUE if the terminal in use can actually display color. (Note: curses uses the American spelling ‘color’, instead of the Canadian/British spelling ‘colour’. If you’re used to the British spelling, you’ll have to resign yourself to misspelling it for the sake of these functions.)

The curses library maintains a finite number of color pairs, containing a foreground (or text) color and a background color. You can get the attribute value corresponding to a color pair with the color_pair() function; this can be bitwise-OR’ed with other attributes such as A_REVERSE, but again, such combinations are not guaranteed to work on all terminals.

一个样例，用 1 号颜色对显示一行文本：

stdscr.addstr("Pretty text", curses.color_pair(1))
stdscr.refresh()

As I said before, a color pair consists of a foreground and background color. start_color() initializes 8 basic colors when it activates color mode. They are: 0:black, 1:red, 2:green, 3:yellow, 4:blue, 5:magenta, 6:cyan, and 7:white. The curses module defines named constants for each of these colors: curses.COLOR_BLACK, curses.COLOR_RED, and so forth.

The init_pair(n, f, b) function changes the definition of color pair n, to foreground color f and background color b. Color pair 0 is hard-wired to white on black, and cannot be changed.

Let’s put all this together. To change color 1 to red text on a white background, you would call:

curses.init_pair(1, curses.COLOR_RED, curses.COLOR_WHITE)

When you change a color pair, any text already displayed using that color pair will change to the new colors. You can also display new text in this color with:

stdscr.addstr(0,0, "RED ALERT!", curses.color_pair(1))

Very fancy terminals can change the definitions of the actual colors to a given RGB value. This lets you change color 1, which is usually red, to purple or blue or any other color you like. Unfortunately, the Linux console doesn’t support this, so I’m unable to try it out, and can’t provide any examples. You can check if your terminal can do this by calling can_change_color(), which returns TRUE if the capability is there. If you’re lucky enough to have such a talented terminal, consult your system’s man pages for more information.

用户输入¶

The curses library itself offers only very simple input mechanisms. Python’s support adds a text-input widget that makes up some of the lack.

The most common way to get input to a window is to use its getch() method. getch() pauses and waits for the user to hit a key, displaying it if echo() has been called earlier. You can optionally specify a coordinate to which the cursor should be moved before pausing.

It’s possible to change this behavior with the method nodelay(). After nodelay(1), getch() for the window becomes non-blocking and returns curses.ERR (a value of -1) when no input is ready. There’s also a halfdelay() function, which can be used to (in effect) set a timer on each getch(); if no input becomes available within a specified delay (measured in tenths of a second), curses raises an exception.

The getch() method returns an integer; if it’s between 0 and 255, it represents the ASCII code of the key pressed. Values greater than 255 are special keys such as Page Up, Home, or the cursor keys. You can compare the value returned to constants such as curses.KEY_PPAGE, curses.KEY_HOME, or curses.KEY_LEFT. Usually the main loop of your program will look something like this:

while 1:
    c = stdscr.getch()
    if c == ord('p'):
        PrintDocument()
    elif c == ord('q'):
        break  # Exit the while()
    elif c == curses.KEY_HOME:
        x = y = 0

The curses.ascii module supplies ASCII class membership functions that take either integer or 1-character-string arguments; these may be useful in writing more readable tests for your command interpreters. It also supplies conversion functions that take either integer or 1-character-string arguments and return the same type. For example, curses.ascii.ctrl() returns the control character corresponding to its argument.

There’s also a method to retrieve an entire string, getstr(). It isn’t used very often, because its functionality is quite limited; the only editing keys available are the backspace key and the Enter key, which terminates the string. It can optionally be limited to a fixed number of characters.

curses.echo()            # Enable echoing of characters

# Get a 15-character string, with the cursor on the top line
s = stdscr.getstr(0,0, 15)

The Python curses.textpad module supplies something better. With it, you can turn a window into a text box that supports an Emacs-like set of keybindings. Various methods of Textbox class support editing with input validation and gathering the edit results either with or without trailing spaces. See the library documentation on curses.textpad for the details.