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Python: How to write multiple strings in one line?

问题描述:

I've started to learn Python with LPTHW and I've gotten to exercise 16:

http://learnpythonthehardway.org/book/ex16.html

And feel like an idiot because I can't figure out one of the seemingly simple "extra credit" assignments that wants the following:

target.write(line1)

target.write('\n')

target.write(line2)

target.write('\n')

target.write(line3)

target.write('\n')

To be condensed to one line of code. I've tried some of the following:

target.write(line1 \n, line2 \n, line3 \n)

Or:

target.write('line1 \n, line2 \n, line3 \n')

Or:

target.write(%r \n, %r \n, %r \n) % (line1, line2, line3)

I just can't get it to rewrite the line1, line2, and line3 strings all in the same line. And I've tried various other combinations with and without commas, quotes, etc. I keep getting varying errors, like Invalid Syntax or that I have too many arguments.

Any help (and an explanation behind it) would be greatly appreciated. Thanks!

网友答案:
target.write(line1 \n, line2 \n, line3 \n)

'\n' only make sense inside a string literal. Without the quotes, you don't have string literals.

target.write('line1 \n, line2 \n, line3 \n')

Ok, now everything is a string literal. But you want line1, line2, line3 to not be string literals. You need those as python expressions to refer the variables in question. Basically, you have to put quotes around strings that are actually text like "\n" but not around variables. If you did that, you might have gotten something like:

target.write(line1 '\n' line2 '\n' line3 '\n')

What is 2 2? It's nothing. You have to specify to python how to combine the two pieces. So you can have 2 + 2 or 2 * 2 but 2 2 doesn't make any sense. In this case, we use add to combine two strings

target.write(line + '\n' + line2 + '\n' + line3 + '\n')

Moving on,

target.write(%r \n, %r \n, %r \n) % (line1, line2, line3)

Again \n only makes sense inside a string literal. The % operator when used to produce strings takes a string as its left side. So you need all of that formatting detail inside a string.

target.write('%r \n', '%r \n', '%r \n') % (line1, line2, line3)

But that produce 3 string literals, you only want one. If you did this, write complained because it excepts one string, not 3. So you might have tried something like:

target.write('%r \n%r \n%r \n') % (line1, line2, line3)

But you want to write the line1, line2, line3 to the file. In this case, you are trying to the formatting after the write has already finished. When python executes this it will run the target.write first leaving:

None % (line1, line2, line3)

Which will do nothing useful. To fix that we need to to put the % () inside the .write()

target.write('%r\n%r\n%r\n' % (line1, line2, line3))
网友答案:

Your last try looks promising. It should look like:

"%s \n %s \n %s" % (line1, line2, line3)

this applies the operator % to a string (with 3 %s placeholders) and a tuple of values to substitute (here, strings). The result is the formatted string.

So you'd need to wrap that in the function which takes the result:

target.write("%s \n %s \n %s" % (line1, line2, line3) )
网友答案:

Here's one way:

target.write(line1 + '\n' + line2 + '\n' + line3 + '\n')

The reason the following doesn't work

target.write(line1 \n, line2 \n, line3 \n)

Is that line1 is a variable (note it's not quoted) but '\n' is a string literal (since it's in quotes). The addition operator is overloaded for strings to concatenate (combine) them.

The reason this doesn't work:

target.write('line1 \n, line2 \n, line3 \n')

Is because line1 is a variable. When you put it in quotes, it's no longer treated as a variable.

网友答案:

below line works for me,

 target.write(line1 + line + line2 + line + line3 + line)

Before that i added

 line = '\n'

my code like:

 from sys import argv
 script, filename = argv
 print 'Appending process starts on: %r' % filename
 target = open(filename, 'a')
 print 'Enter the contents:\t'
 line1 = raw_input('Next:\t')
 line2 = raw_input('Next:\t')
 line3 = raw_input('Next:\t')
 line = '\n'
 target.write(line1 + line + line2 + line + line3 + line)
 print 'Thank you !'
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