\b backspace character use

[Martin Gerard]

when running the code below in the Python IDLE Shell I get a small white vertical rectangle where the backspace character is. Why is this? I thought that the \b character moved text to the left. In Spyder development environment I get Adc. Could somebody explain in layman terms why this is happening? Thank in advance.  

print("Abc\bd")

[Tim Holloway]

Backspace is a control character. What that actually means is that it indicates that a hardware control operation is to be performed — if the hardware supports it.

On old-fashioned typewriter-style devices, BS would cause the type carrier to physically move 1 space back, and was often used to overtype text, either to make text bolder or to create special characters, like umlauts over vowels and overstrikes/underlines. This was also often done on character printers such as dot-matrix printers.

On CRTs, a backspace moved the character cursor back one space. Most CRTs could not "overtype", so the next output character would simply replace the previous character on the display.

Modern-day outputs are a bigger problem, though. Proportional fonts are common and you cannot simply "back up 1 space" when a "W" and an "I" take up different amounts of horizontal space.

So what you actually have here is that instead of using the BS character to control hardware, the display device is attempting to render that character code (0x08) as a displayable character. Since almost no fonts define character glyphs for control characters, the "no glyph available" glyph is rendered. Typically that glyph will be a hollow rectangular box or a vertical line of some sort.

[Martin Gerard]

Thank you for the explanation Tim. Much appreciated.