Python 实用宝典

Question 1

I would like to create a string buffer to do lots of processing, format and finally write the buffer in a text file using a C-style sprintf functionality in Python. Because of conditional statements, I can’t write them directly to the file.

e.g pseudo code:

sprintf(buf,"A = %d\n , B= %s\n",A,B)
/* some processing */
sprint(buf,"C=%d\n",c)
....
...
fprintf(file,buf)

So in the output file we have this kind of o/p:

A= foo B= bar
C= ded
etc...

Edit, to clarify my question:
buf is a big buffer contains all these strings which have formatted using sprintf. Going by your examples, buf will only contain current values, not older ones. e.g first in buf I wrote A= something ,B= something later C= something was appended in the same buf, but in your Python answers buf contains only last value, which is not I want – I want buf to have all the printfs I have done since the beginning, like in C.

Question 2

Python has a % operator for this.

>>> a = 5
>>> b = "hello"
>>> buf = "A = %d\n , B = %s\n" % (a, b)
>>> print buf
A = 5
 , B = hello

>>> c = 10
>>> buf = "C = %d\n" % c
>>> print buf
C = 10

See this reference for all supported format specifiers.

You could as well use format:

>>> print "This is the {}th tome of {}".format(5, "knowledge")
This is the 5th tome of knowledge

Question 3

If I understand your question correctly, format() is what you are looking for, along with its mini-language.

Silly example for python 2.7 and up:

>>> print "{} ...\r\n {}!".format("Hello", "world")
Hello ...
 world!

For earlier python versions: (tested with 2.6.2)

>>> print "{0} ...\r\n {1}!".format("Hello", "world")
Hello ...
 world!

Question 4

I’m not completely certain that I understand your goal, but you can use a StringIO instance as a buffer:

>>> import StringIO 
>>> buf = StringIO.StringIO()
>>> buf.write("A = %d, B = %s\n" % (3, "bar"))
>>> buf.write("C=%d\n" % 5)
>>> print(buf.getvalue())
A = 3, B = bar
C=5

Unlike sprintf, you just pass a string to buf.write, formatting it with the % operator or the format method of strings.

You could of course define a function to get the sprintf interface you’re hoping for:

def sprintf(buf, fmt, *args):
    buf.write(fmt % args)

which would be used like this:

>>> buf = StringIO.StringIO()
>>> sprintf(buf, "A = %d, B = %s\n", 3, "foo")
>>> sprintf(buf, "C = %d\n", 5)
>>> print(buf.getvalue())
A = 3, B = foo
C = 5

Question 5

Use the formatting operator %:

buf = "A = %d\n , B= %s\n" % (a, b)
print >>f, buf

Question 6

You can use string formatting:

>>> a=42
>>> b="bar"
>>> "The number is %d and the word is %s" % (a,b)
'The number is 42 and the word is bar'

But this is removed in Python 3, you should use “str.format()”:

>>> a=42
>>> b="bar"
>>> "The number is {0} and the word is {1}".format(a,b)
'The number is 42 and the word is bar'

Question 7

To insert into a very long string it is nice to use names for the different arguments, instead of hoping they are in the right positions. This also makes it easier to replace multiple recurrences.

>>> 'Coordinates: {latitude}, {longitude}'.format(latitude='37.24N', longitude='-115.81W')
'Coordinates: 37.24N, -115.81W'

Taken from Format examples, where all the other Format-related answers are also shown.

Question 8

This is probably the closest translation from your C code to Python code.

A = 1
B = "hello"
buf = "A = %d\n , B= %s\n" % (A, B)

c = 2
buf += "C=%d\n" % c

f = open('output.txt', 'w')
print >> f, c
f.close()

The % operator in Python does almost exactly the same thing as C’s sprintf. You can also print the string to a file directly. If there are lots of these string formatted stringlets involved, it might be wise to use a StringIO object to speed up processing time.

So instead of doing +=, do this:

import cStringIO
buf = cStringIO.StringIO()

...

print >> buf, "A = %d\n , B= %s\n" % (A, B)

...

print >> buf, "C=%d\n" % c

...

print >> f, buf.getvalue()

Question 9

If you want something like the python3 print function but to a string:

def sprint(*args, **kwargs):
    sio = io.StringIO()
    print(*args, **kwargs, file=sio)
    return sio.getvalue()

>>> x = sprint('abc', 10, ['one', 'two'], {'a': 1, 'b': 2}, {1, 2, 3})
>>> x
"abc 10 ['one', 'two'] {'a': 1, 'b': 2} {1, 2, 3}\n"

or without the '\n' at the end:

def sprint(*args, end='', **kwargs):
    sio = io.StringIO()
    print(*args, **kwargs, end=end, file=sio)
    return sio.getvalue()

>>> x = sprint('abc', 10, ['one', 'two'], {'a': 1, 'b': 2}, {1, 2, 3})
>>> x
"abc 10 ['one', 'two'] {'a': 1, 'b': 2} {1, 2, 3}"

Question 10

Something like…

greetings = 'Hello {name}'.format(name = 'John')

Hello John

Question 11

Take a look at “Literal String Interpolation” https://www.python.org/dev/peps/pep-0498/

I found it through the http://www.malemburg.com/

Question 12

Two approaches are to write to a string buffer or to write lines to a list and join them later. I think the StringIO approach is more pythonic, but didn’t work before Python 2.6.

from io import StringIO

with StringIO() as s:
   print("Hello", file=s)
   print("Goodbye", file=s)
   # And later...
   with open('myfile', 'w') as f:
       f.write(s.getvalue())

You can also use these without a ContextMananger (s = StringIO()). Currently, I’m using a context manager class with a print function. This fragment might be useful to be able to insert debugging or odd paging requirements:

class Report:
    ... usual init/enter/exit
    def print(self, *args, **kwargs):
        with StringIO() as s:
            print(*args, **kwargs, file=s)
            out = s.getvalue()
        ... stuff with out

with Report() as r:
   r.print(f"This is {datetime.date.today()}!", 'Yikes!', end=':')

Python 实用宝典

类似于sprintf的Python功能

问题：类似于sprintf的Python功能

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