Dictionary-Based String Formatting Expressions

String formatting also allows conversion targets on the left to refer to the keys in a dictionary on the right and fetch the corresponding values. I haven’t told you much about dictionaries yet, so here’s an example that demonstrates the basics:

>>> "%(n)d %(x)s" % {"n":1, "x":"spam"}
'1 spam'

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Here, the (n) and (x) in the format string refer to keys in the dictionary literal on the right and fetch their associated values. Programs that generate text such as HTML or XML often use this technique—you can build up a dictionary of values and substitute them all at once with a single formatting expression that uses key-based references:

>>> reply = """                               # Template with substitution targets
Greetings...
Hello %(name)s!
Your age squared is %(age)s
"""
>>> values = {'name': 'Bob', 'age': 40}       # Build up values to substitute
>>> print(reply % values)                     # Perform substitutions

Greetings...
Hello Bob!
Your age squared is 40

This trick is also used in conjunction with the vars built-in function, which returns a dictionary containing all the variables that exist in the place it is called:

>>> food = 'spam'
>>> age = 40
>>> vars()
{'food': 'spam', 'age': 40, ...many more... }

When used on the right of a format operation, this allows the format string to refer to variables by name (i.e., by dictionary key):

>>> "%(age)d %(food)s" % vars()
'40 spam'

We’ll study dictionaries in more depth in Chapter 8. See also Chapter 5 for examples that convert to hexadecimal and octal number strings with the %x and %o formatting target codes.