问题
Expected:
>>> print "I print\b\b Backspace\b\b!"
I pri Backspa!
>>> print "I print\b\b Backspace\b\b\b!"
I pri Backsp!
Observed:
>>> print "I print\b\b Backspace\b\b!"
I pri Backspa!e
>>> print "I print\b\b Backspace\b\b\b!"
I pri Backsp!ce
Why does is 'e' and 'ce' not erased and '!' inserted?
回答1:
The \b also termed as back space moves back the cursor by 1.
The backspace doesn't delete anything, it moves the cursor to the left and it gets covered up by what you write afterwards.
Let's understand this with your example:
"I print\b\b Backspace\b\b!" # original string passed to "print"
# ^^ * * ^^ * *
# 12 1 2 34 3 4
After executing
*1and*2, cursor comes at^1. Hence,^1is replaced by space' 'and^2replaced byB(characters following\b)After executing
*3, and*4, cursor comes at^3and is replaced by!. Since there was nothing after!,^4remains as it is, else would have be replaced by the next character.
Hence the resultant content that is printed on the screen is as:
I pri Backspa!e
回答2:
You didn't erase them; you merely backspaced. Going forward will overwrite the previous characters, but backspace doesn't erase as it backs up. You would want
print "I print\b\b Backspace\b\b !"
... to see ...
I pri Backspa !
If you want the "full effect", you have to backspace and overwrite wtih spaces as you back up ... then you can move forward. Something like
print "Backspace" + 2*"\b \b" + "!"
You can use the multiplier as many times as you wish; it's a small motif. The above line displays
Backspa!
回答3:
The \b character moves the carriage back by one space, similar to how \r moves the carriage all the way back to the beginning of the line. If you don't overwrite the characters you backed up over, they will still be there.
>>> print "I print\b\b Backspace\b\b!"
I pri Backspa!e
>>> print "I print\b\b Backspace\b\b! "
I pri Backspa!
来源：https://stackoverflow.com/questions/40251434/please-explain-this-unexpected-b-backspace-behavior