Format

It’s time we look at how to interpolate data values into strings—in other words, put the values inside the strings—using various formats ( place holder).

Old style with %

simply sequence is a % followed by a letter indicating the data type to be formatted.

%s – string
%d- decimal integer
%x – hex integer
%o – octal integer
%f – decimal float
%e – exponential float
%g – decimal or exponential float
%% – a literal %

A float:

 >>> '%s' % 7.03
 '7.03'
 >>> '%f' % 7.03
 '7.030000'
 >>> '%e' % 7.03
 '7.030000e+00'
 >>> '%g' % 7.03
 '7.03'

An integer and a literal %:

 >>> '%d%%' % 100
 '100%'

Some string and integer interpolation:

 >>> actor = 'Richard Gere'
 >>> cat = 'Chester'
 >>> weight = 28
 >>> "My wife's favorite actor is %s" % actor
 "My wife's favorite actor is Richard Gere"
 >>> "Our cat %s weighs %s pounds" % (cat, weight)
 'Our cat Chester weighs 28 pounds'

That %s inside the string means to interpolate a string. The number of % appearances in the string needs match the number of data items after the %.

Multiple data must be grouped into a tuple (bounded by parentheses, separated by commas) such as:

print("My wife's favorite actor is %s and his cat is %s,\

whose weight is %d" %(actor,cat,weight))

the output will be:

My wife's favorite actor is Richard Gere and his cat is Chester,whose weight is 28

You can add other values between the % and the type specifier to designate minimum and maximum widths, alignment, and  character filling:

1.  Set a minimum field width of 10 characters for each variable, and align them to the right, filling unused spots on the left with  spaces.

 >>> n = 42
 >>> f = 7.03
 >>> s = 'string cheese'

 >>>'%10d %10f %10s' % (n, f, s)
 '        42   7.030000 string cheese'

2.  Use the same field width, but align to the left (minus number):

>>> '%-10d %-10f %-10s' % (n, f, s)
 '42         7.030000   string cheese'

3.   number after the decimal: It means maximum character number for the string, but means the number of digit after the decimal point for floating number . E.g. same field width(6), maximum character width of 2, and aligned to the right. This setting truncates the string, and limits the float to 2 digits after the  decimal point:

Now we make the weight of the cat little bit of

actor='Richard Gere'

cat='Chester'

weight=28.121313

print("My wife's favorite actor is %s and his cat is %10.5s,whose weight is %8.2f" %(actor,cat,weight))

output:

My wife's favorite actor is Richard Gere and his cat is      Chest,whose weight is   28.12

4. get the field widths from arguments rather than hard-coding them:

>>> '%*.*d %*.*f %*.*s' % (10, 4, n, 10, 4, f, 10, 4, s)
 '      0042     7.0300       stri'

Print

If we assign this string to the keyword parameter “sep” of the print function:

>>> q = 459
>>> p = 0.098
>>> print(q, p, p * q)
459 0.098 44.982
>>> print(q, p, p * q, sep=",")
459,0.098,44.982
>>> print(q, p, p * q, sep=" :-) ")
459 :-) 0.098 :-) 44.982

New style formatting with {} and format

The new format method was added in Python 2.6 , however the old format is still supported. The general form of this method looks like this:

template.format(p0, p1, ..., k0=v0, k1=v1, ...)

The template (or format string) is a string which contains one or more format codes (fields to be replaced) embedded in constant text. The “fields to be replaced” are surrounded by curly braces {}.

eg.

>>n=42
>>f=7.03
>>s='string cheese'
>> '{} {} {}'.format(n, f, s)
 '42 7.03 string cheese'

1.  With new-style, you can specify the order:

>> '{2} {0} {1}'.format(f, s, n)
 '42 7.03 string cheese'

2. The list of arguments starts with zero or more positional argument.

The arguments can be a dictionary or named arguments, and the specifiers can include their names:

>> '{n} {f} {s}'.format(n=42, f=7.03, s='string cheese')
 '42 7.03 string cheese'

3. Add a data temporarily, In the following example, {0} is the entire dictionary, whereas {1} is the string ‘other’ that follows the dictionary:

>> '{0[n]} {0[f]} {0[s]} {1}'.format(d, 'other')
 '42 7.03 string cheese other'

4. Specify the data type after a “:” .

To specify n as dicemal(d), f as floating(f), s as string data(s).

>> '{0:d} {1:f} {2:s}'.format(n, f, s)
 '42 7.030000 string cheese'

5. Similar to old style, add the number to specify the minimum field width, maximum character width, alignment:

Example:

6. The alignment expression is different :

>>> x = 378
>>> print("The value is {:06d}".format(x))
The value is 000378
>>> x = -378
>>> print("The value is {:06d}".format(x))
The value is -00378

>>> print("The value is {:,}".format(x))
The value is 78,962,324,245
>>> print("The value is {0:6,d}".format(x))
The value is 5,897,653,423
>>> x = 5897653423.89676
>>> print("The value is {0:12,.3f}".format(x))
The value is 5,897,653,423.897

7. The final option is the fill character. If you want something other than spaces to pad your output fields, put it right after the :, before any alignment (<, >, ^) or width specifiers.

>> '{0:!^20s}'.format('BIG SALE')
 '!!!!!!BIG SALE!!!!!!'