________________________________
> Date: Tue, 14 Sep 2010 22:15:40 +0100
> From: wpr...@gmail.com
> To: tutor@python.org
> Subject: Re: [Tutor] FW: wierd replace problem
>
>
>
> On 14 September 2010 21:10, Roelof Wobben
>> wrote:
> I understand it but I try to understand why in a file there is this
> 'word python makes a "'word.
>
> Python doesn't change what it reads from the file. However, depending
> on how you ask Python to tell you what it's read (or what the contents
> of a string variable is), it might display it quoted, and/or include
> escape charaters, or not as the case may be. If what you've read from
> the file contains quotes, then obviously you need to be careful to not
> mistake Python's quoting of the value of a string as being *part* of
> that string. Neither must you mistake the escape character (if
> applicable) from being actually part of the string.
>
> For example, consider the following exchange in the Python shell
> (please try all of this yourself and experiment):
>
>>>> s = 'blah'
>>>> s
> 'blah'
>>>> print s
> blah
>>>>
>
> I assign the value of 'blah' to the string s. So far simple enough.
> Obviosuly the quotes used int the assignment of the string does not
> form part of the string itself. Their utility is only to delineate to
> Python the start of the string, and the end of the string.
>
> In the next line I ask Python to evaluate the expression s, which it
> duly reporst as 'blah'. Again, it's using normal Python convention to
> format the data as a string, because that's what s is, a string
> object. But the quotes are formatting, they're not really part of the
> string.
>
> In the next line I ask Python to print s. Now, the true content of s
> is printed as it is, and hence you can see that the quotes are not part
> of the string.
>
> Now consider the following exchange in the Python shell where I open a
> file and write some text to it to prove this point:
>>>> f = open('test.txt', 'w+')
>>>> f.write('blah')
>>>> f.close()
>>>> import os
>>>> os.system('notepad test.txt')
>
> The last line above opens the text file test.txt in Notepad so you can
> see the contents. As you can see, no quotes or anything else. Now,
> while open, suppose we put a single quote in the file, so it reads:
> 'blah
> ...and suppose we then save it and exit notepad so you're back in the
> Python shell. Then we do:
>
>>>> f=open('test.txt','r+')
>>>> s=f.read()
>>>> f.close()
>>>> s
> "'blah"
>
> Now I've read the contents of the file back into a string variable s,
> and asked Python to evaluate (output) this string object.
>
> Notice, Python is now formatting the string with *doube* quotes
> (previously it defaulted to single quotes) to avoid having to escape
> the single quote that forms part of the string. If Python had used
> single quotes instead, then there would've been an ambiguity with the
> single quote that's part of the string and so it would've had to escape
> that too. So consequently it formats the string with double quotes,
> which is valid Python syntax and avoids the backslash. (Stating the
> obvious, strings can be quoted with double or single quotes.) As
> before, the double quotes, as with the single quotes earlier, are not
> part of the string. They are merely formatting since Python is being
> asked to display a string and hence it must indicate the start and end
> of the string with suitable quote characters.
>
> Now, as before do:
>
>>>> print s
> 'blah
>
> As before, with print you see the contents of the string as it is (and
> as indeed it is also in the file that you saved). Just the single quote
> you added at the front of Blah. No double or single quotes or anything
> else.
>
> Now finally, let's try something a bit more elaborate. Do again:
>
>>>> os.system('notepad test.txt')
>
> Then put into the file the following 2 lines of text (notice the file
> now contains 2 lines, and both single and double quotes...):
> +++"+++This line is double quoted in the file and the quotes have +
> symbols around them.+++"+++
> ---'---This line is single quoted in the file and the quotes have -
> symbols around them.---'---
>
> Save it, exit Notepad, then do:
>>>> f=open('test.txt', 'r+')
>>>> s=f.read()
>>>> f.close()
>>>> s
> '+++"+++This line is double quoted in the file and the quotes have +
> symbols around them.+++"+++\n---\'---This line is single quoted in the
> file and the quotes have - symbols around them.---\'---\n'
>>>> print s
> +++"+++This line is double quoted in the file and the quotes have +
> symbols around them.+++"+++
> ---'---This line is single quoted in the file and the quotes have -
> symbols around them.---'---
>
> Notice we read both lines in the file into one single string. See how
> Python formats that as a string object, and escapes not only the single
> quotes but also the line break characters (\n). See also when Python
> is asked to "print" the string, you can see the escape characters
> really there. See what's happened? Do you understand why?
>
> Walter
>
>
>
>
>
>
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hello Walter,
I think I understand why.
Python formats the string with a '.
Because there are ' and " in the string Python has to use escape characters
because otherwise Python can never know when the string begins and ends. When
you only use ' or " then python uses the other so I knows when a string begins
and end.
Roelof
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