Advanced String Formatting Expressions

For more advanced type-specific formatting, you can use any of the conversion type codes listed in Table 7-4 in formatting expressions; they appear after the % character in substitution targets. C programmers will recognize most of these because Python string formatting supports all the usual C printf format codes (but returns the result, instead of displaying it, like printf). Some of the format codes in the table provide alternative ways to format the same type; for instance, %e, %f, and %g provide alternative ways to format floating-point numbers.

Table 7-4. String formatting type codes

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Code

Meaning

s

String (or any object’s str(X) string)

r

s, but uses repr, not str

c

Character

d

Decimal (integer)

i

Integer

u

Same as d (obsolete: no longer unsigned)

o

Octal integer

x

Hex integer

X

x, but prints uppercase

e

Floating-point exponent, lowercase

E

Same as e, but prints uppercase

f

Floating-point decimal

F

Floating-point decimal

g

Floating-point e or f

G

Floating-point E or F

%

Literal %

In fact, conversion targets in the format string on the expression’s left side support a variety of conversion operations with a fairly sophisticated syntax all their own. The general structure of conversion targets looks like this:

%[(name)][flags][width][.precision]typecode

The character type codes in Table 7-4 show up at the end of the target string. Between the % and the character code, you can do any of the following: provide a dictionary key; list flags that specify things like left justification (−), numeric sign (+), and zero fills (0); give a total minimum field width and the number of digits after a decimal point; and more. Both width and precision can also be coded as a * to specify that they should take their values from the next item in the input values.

Formatting target syntax is documented in full in the Python standard manuals, but to demonstrate common usage, let’s look at a few examples. This one formats integers by default, and then in a six-character field with left justification and zero padding:

>>> x = 1234
>>> res = "integers: ...%d...%−6d...%06d" % (x, x, x)
>>> res
'integers: ...1234...1234  ...001234'

The %e, %f, and %g formats display floating-point numbers in different ways, as the following interaction demonstrates (%E is the same as %e but the exponent is uppercase):

>>> x = 1.23456789
>>> x
1.2345678899999999

>>> '%e | %f | %g' % (x, x, x)
'1.234568e+00 | 1.234568 | 1.23457'

>>> '%E' % x
'1.234568E+00'

For floating-point numbers, you can achieve a variety of additional formatting effects by specifying left justification, zero padding, numeric signs, field width, and digits after the decimal point. For simpler tasks, you might get by with simply converting to strings with a format expression or the str built-in function shown earlier:

>>> '%−6.2f | %05.2f | %+06.1f' % (x, x, x)
'1.23   | 01.23 | +001.2'

>>> "%s" % x, str(x)
('1.23456789', '1.23456789')

When sizes are not known until runtime, you can have the width and precision computed by specifying them with a * in the format string to force their values to be taken from the next item in the inputs to the right of the % operator—the 4 in the tuple here gives precision:

>>> '%f, %.2f, %.*f' % (1/3.0, 1/3.0, 4, 1/3.0)
'0.333333, 0.33, 0.3333'

If you’re interested in this feature, experiment with some of these examples and operations on your own for more information.