Best way to transfer print data in python

Question

I went through http://web2py.com/book/default/chapter/02 and found this:

>>> print 'number is ' + str(3)
number is 3
>>> print 'number is %s' % (3)
number is 3
>>> print 'number is %(number)s' % dict(number=3)
number is 3

It was given that The last notation is more explicit and less error prone, and is to be preferred.

I wonder what the advantage of using the latest notation is. Would it not be an overhead for productivity?

Answer 1

>>> print 'number is ' + str(3)
number is 3

This is by far the worst solution and can cause problems if you make a beginner error "Value of obj: " + objwhere objit is not a string or unicode object. For many concatenations it is not readable at all - it looks like something like echo "<p>Hello ".$username."!</p>";in PHP (it can turn out to be arbitrarily ugly).

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>>> "number is {0}".format(3)
'number is 3'

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>>> print 'number is %(number)s' % dict(number=3)
number is 3

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>>> "number is {number}".format(number=3)
'number is 3'

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Answer 2

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>>> print 'number is %s %s' % (3,4)
number is 3 4
>>> print 'number is %s %s' % (4,3)
number is 4 3
>>> print 'number is %(number)s %(two)s' % dict(number=3, two=4)
number is 3 4
>>> print 'number is %(number)s %(two)s' % dict(two=4, number=3)
number is 3 4
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Answer 4

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The second method is the easiest method, and this is what you usually use in most programs. This is best used when the format string is immediate, for example. 'values: %s %s %s' % (a, b, c)instead of taking from a variable, for example. fmt % (a, b, c).

The first concatenation is almost never useful, except, perhaps, if you create a list in cycles:

s = ''
for x in l:
    s += str(x)

however, in this case, it is generally better and faster to use str.join ():

s = ''.join(str(x) for x in l)