CODEWITHSUNDEEP
pythonstring-formatting
nizam Khan
nizam Khan

Reputation: 1

Don't understand string formatting expressions

I am havig some doubt regarding the string formatting expressions in python:

The general syntax of the string formatting expression is

%[keyname][flags][width][.precision]typecode

I am having couple of doubt in this and these are:

  1. What does keyname stands for?
  2. Can we use keyword arguments instead of variable name and dictionary?

For example:

x="%(z)d is equal to" %{"z":1}  # This expression gives me the required output.

but when I try this one:

x="%(z)d is equal to" %("z"=1)  # Causes error. I am getting confused with this.

similarly when I use like this:

x="%(z)d is equal to" %dict("z"=1)

It also shows an error. Why?

Upvotes: 0

Views: 58

Answers (1)

BoarGules
BoarGules

Reputation: 16942

Keyname means the name in parentheses, for example z in %(z)d. It is called keyname because "z" is the dictionary key that will return the value you want substituted in your string, in this case 1.

You are confusing two ways to specify a dictionary.

One is with braces { }, for which the syntax is {"z": 1}. You can specify the key (here z) as a constant or as a variable and the variable can contain any hashable value: tuples qualify, lists don't, recursively.

So, as you have found out, this is valid:

x = "%(z)d is equal to" % {"z":1}

The other way is to call the dict constructor, for which the syntax is dict(z=1). But here the key must be a valid Python identifier because in this syntax you are passing z as a keyword parameter to dict().

In this syntax the equivalent of the first valid example is:

x ="%(z)d is equal to" % dict(z=1)

You are getting errors on the other lines because

  • ("z"=1) does not specify a dict because it has neither {...} nor a dict() constructor call.

  • dict("z"=1) is mixing up the first syntax (key is a constant, use { } and colon instead) with the second syntax (key can't be a constant: it needs to be an identifier that you provide as a keyword parameter to the dict() call).

Upvotes: 1

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