{article Dive into Python}{title} {text}{/article}

Python supports formatting values into strings. Although this can include very complicated expressions, the most basic usage is to insert values into a string with the %s placeholder.

1. print ("Today's stock price: %f" % 50.4625)
2. print ("Today's stock price: %.2f" % 50.4625)
3. print ("Today's stock price: %+.2f" % 1.5)

1. The %f string formatting option treats the value as a decimal, and prints it to six decimal places.
2. The ".2" modifier of the %f option truncates the value to two decimal places.
3. You can even combine modifiers. Adding the + modifier displays a plus or minus sign before the value. Note that the ".2" modifier is still in place, and is padding the value to exactly two decimal places.

How to print a string

>>> string = "# How to print a string."
>>> print (string)
# How to print a string.

>>> print string
SyntaxError: invalid syntax

k = "uid"
v = "sa"
"%s=%s" % (k, v)

Concatenating

>>> uid = "sa"
>>> pwd = "secret"
1: >>> print pwd + " is not a good password for " + uid
SyntaxError: invalid syntax
>>> print (pwd + " is not a good password for " + uid)
secret is not a good password for sa
>>> print "%s is not a good password for %s" % (pwd, uid)
SyntaxError: invalid syntax
>>> print ("%s is not a good password for %s") % (pwd, uid)
%s is not a good password for %s
Traceback (most recent call last):
File "<pyshell#8>", line 1, in <module>
print ("%s is not a good password for %s") % (pwd, uid)
TypeError: unsupported operand type(s) for %: 'NoneType' and 'tuple'
2: >>> print ("%s is not a good password for %s" % (pwd, uid))
secret is not a good password for sa
>>> userCount = 6
3: 4: >>> print ("Users connected: %d" % (userCount, ))
Users connected: 6
5: >>> print ("Users connected: " + userCount)
Traceback (most recent call last):
File "<pyshell#13>", line 1, in <module>
print ("Users connected: " + userCount)
TypeError: Can't convert 'int' object to str implicitly
>>>

1. + is the string concatenation operator.
2. 
   In this trivial case, string formatting accomplishes the same result as concatentation.
3. 
   (userCount, ) is a tuple with one element. Yes, the syntax is a little strange, but there's a good reason for
   it: it's unambiguously a tuple. In fact, you can always include a comma after the last element when defining a
   list, tuple, or dictionary, but the comma is required when defining a tuple with one element. If the comma
   weren't required, Python wouldn't know whether (userCount) was a tuple with one element or just the value
   of userCount.
4. 
   String formatting works with integers by specifying %d instead of %s.
5. 
   Trying to concatenate a string with a non−string raises an exception. Unlike string formatting, string
   concatenation works only when everything is already a string.

As with printf in C, string formatting in Python is like a Swiss Army knife. There are options galore, and modifier
strings to specially format many different types of values.

Numbers

>>> print("{:.2f}".format(3.1415926)) 2 decimal places
3.14
>>> print("{:+.2f}".format(+3.1415926)) 2 decimal places with sign
+3.14
>>> print("{:+.2f}".format(-1)) 2 decimal places with sign
-1.00
>>> print("{:.0f}".format(2.71828)) No decimal places
3
>>> print("{:0>2d}".format(5)) Pad number with zeros (left padding, width 2)
05
>>> print("{:x<4d}".format(5)) Pad number with x's (right padding, width 4)
5xxx
>>> print("{:x<4d}".format(10)) Pad number with x's (right padding, width 4)
10xx
>>> print("{:,}".format(1000000)) Number format with comma separator
1,000,000
>>> print("{:.2%}".format(0.25)) Format percentage
25.00%
>>> print("{:.2e}".format(1000000000)) Exponent notation
1.00e+09
>>> print("{:10d}".format(13)) Right aligned (default, width 10)
13
>>> print("{:<10d}".format(13)) Left aligned (width 10)
13
>>> print("{:^10d}".format(13)) Center aligned (width 10)
13