One of the commonest string operations is Python's format operator. Here, we want to substitute variable values into a fixed string.
For example, suppose your program wants to report how
many bananas you have, and you have an
nBananas that contains the
actual banana count, and you want to print a string
something like “We have 27 bananas” if
nBananas has the value 27. This is how
you do it:
>>> nBananas = 27 >>> "We have %d bananas." % nBananas 'We have 27 bananas.'
In general, when a string value appears on the left side
of the “
operator, that string is called the format
string. Within a format string, the percent
%” has special
meaning. In the example above, the “
%d” part means that an integer value will
be substituted into the format string at that position.
So the result of the format operator will be a string
containing all the characters from the format string,
except that the value on the right of the operator (
27) will replace the “
%d” in the format string.
Here's another example, showing the substitution of a string value.
>>> noSuch = "kiwis" >>> 'We are out of %s today.' % noSuch 'We are out of kiwis today.'
This demonstrates the “
format code, which means that a string value is to be
substituted at that position in the result.
You can substitute more than one value in a format operation, but you must enclose the values to be substituted in parentheses, separated by commas. For example:
>>> caseCount = 42 >>> caseContents = "peaches" >>> print "We have %d cases of %s today." % (caseCount, caseContents) We have 42 cases of peaches today.
So, in general, a format operator has this form:
Wherever a format code starting with “
%” appears in the
string, the corresponding
is substituted for that format
The various format codes have a number of additional
features that let you control how the values are
displayed. For example, the “
%s” format code always produces a value exactly as
long as the string value you provide. But you may
wish to produce a value of a fixed size. To do this, use
a format code of the form “
is the number of
characters you want the result to occupy. Examples:
>>> '%s' % 'soup' 'soup' >>> '%6s' % 'soup' ' soup'
If a string shorter than
characters is formatted using
format code “
%”, spaces are added before the string to
fill the result out to
characters. If you would
prefer that the extra spaces be added after the string
value, use a format code of the form “
>>> '%-6s' % 'soup' 'soup '
By default, the integer format code “
%d” always produces a string that is just
large enough to hold the number. But if you want the
number to occupy exactly
digits, you can use a format
code of the form “
>>> "%d" % 1107 '1107' >>> "%5d" % 1107 ' 1107' >>> '%30d' % 1107 ' 1107' >>> '%2d' % 1107 '1107'
Notice in the last example that when you specify a field size that is too small for the number, Python will not truncate the number; it will take as many characters as needed to properly render the value.
When your number does not fill the field, the default is
to add spaces to the left of the number as needed. If
you would prefer that the extra spaces be added after the
number, use a format code of the form “
>>> '%5d' % 505 ' 505' >>> '%-5d' % 505 '505 '
You can ask Python to use zeroes instead of spaces to
fill extra positions by using a format code of the form
>>> '%5d' % 42 ' 42' >>> '%05d'%42 '00042'
Next we'll examine Python's format code for
float values. In its simplest form, it is just
>>> "%f" % 0.0 '0.000000' >>> "%f" % 1.5 '1.500000' >>> pi = 3.141592653589793 >>> "%f" % pi '3.141593'
By default, the result will show six digits of precision
after the decimal point. To specify
digits of precision, use a
format code of the form “
>>> "%.0f" % pi '3' >>> "%.15f" % pi '3.141592653589793'
You can also specify the total number of characters to be
used in formatting a number. A format code of the form
try to fit the result into
digits after the
>>> "%10f" % pi ' 3.141593' >>> "%5.1f" % pi ' 3.1' >>> "%5.3f" % pi '3.142' >>> "%50.40f" % 5.33333 ' 5.3333300000000001261923898709937930107117'
Notice in the last example above that it is possible for you to produce any number of spurious digits beyond the precision used to specify the number originally! Beware, because those extra digits are utter garbage.
When you specify a precision, the value is rounded to the nearest value with that precision.
>>> "%.1f" % 0.999 '1.0' >>> "%.1f" % 0.99 '1.0' >>> "%.1f" % 0.9 '0.9' >>> "%.1f" % 0.96 '1.0' >>> "%.1f" % 0.9501 '1.0' >>> "%.1f" % 0.9499999 '0.9' >>>
As with the
formats, you can use a negative field size in the
%f format code to cause the value to be
left-aligned in the field.
>>> "%10.2f" % pi ' 3.14' >>> "%-10.2f" % pi '3.14 '
If you would prefer to display a
value using the exponential format, use a format code of
the form “
exponent will always occupy four or five digits depending
on the size of the exponent.
>>> avo = 6.022e23 >>> "%e" % avo '6.022000e+23' >>> "%.3e" % avo '6.022e+23' >>> "%11.4e" % avo ' 6.0220e+23' >>> googol = 1e100 >>> "%e" % googol '1.000000e+100' >>> "%e" % pi '3.141593e+00'