CODEWITHSUNDEEP
python
vipin
vipin

Reputation: 678

python string formatting with keyword arguments

I have a situation like this.

a={'x':'test'}
b="this is a %(a['x'])s
print b % {'test':'testing'}

the desired result is 

>>this is a testing

but it throws an error "ValueError: incomplete format key"

Please suggest me a better way.Thanks in advance.

Upvotes: 3

Views: 370

Answers (7)

Leon
Leon

Reputation: 12491

You can expand a dictionary to named parameters

a = {'test': 'testing'}
b = "this is a {test}".format(**a)
print b

output: this is a testing

The ** in front of the dictionary causes a parameter to be passed to the method which has the name that matches the key and the value matched the value in the dictionary.

Now looking at your code and what you are trying to achieve you are using two dictionaries. So you need to format your string twice.

a={'x':'test'}
b="this is a {{{x}}}".format(**a)
print b
print b.format(**{'test':'testing'})

output:

this is a {test}

this is a testing

The first format creates a new named place holder with the value of key x. The second format will populate the place holder.

Upvotes: 1

aptro
aptro

Reputation: 51

Why showing key error

In your code %(a['x']) will simply convert key argument to "a['x']"

This is equivalent to:

    a={'x':'test'}
    b="this is a %(a['x'])s
    print b % {"a['x']":'testing'}
    "this is a testing"

You can use format or % suggested by other answers

Upvotes: 1

lord63. j
lord63. j

Reputation: 4670

How about using format:

>>> a = {'x': 'test'}
>>> print("this is a {0[x]}".format(a))
this is a test

How to use format:

{field!convertflag:formatspec}

formatspec:

[[fill]align][sign][#][0][width][.precision][typecode]

I'll give you some example:

>>> import sys
>>> print("I'm using python {0.version}".format(sys))
I'm using python 2.7.9 (default, Mar 31 2015, 09:35:58)
[GCC 4.8.1]
>>> print("Hello, {name}".format(name="lord63"))
Hello, lord63
>>> print("{0}, {1}, {0}".format("one", "two"))
one, two, one
>>> print("The fifth: {0[4]}".format(range(1,6)))
5
>>> print("I'm {0[name]}".format(dict(name="lord63", age=20)))
I'm lord63

Upvotes: -1

skyking
skyking

Reputation: 14400

There's more problem with your code than you're telling. The string is missing the trailing ", but also it's not quite clear what you're trying to do. What's the a for?

What you need to do is to put the key in the parentheses, for example:

b="this is a %(test)s"

Or if you want to get the x from a and and then expand you'll need to do something like:

b="this is a %%(%(x)s)s" % a

This will expand the x in a and result in the string in the former example. I used double percent to escape the percent sign.

Upvotes: 0

Iwan1993
Iwan1993

Reputation: 1779

You could do the exchange before inserting into string.

c = { 'test' : 'string' }
y = c[a['x']]

Then simply "mystring %s" % y

If you dont want to exchange the values before you could use

("{0[%s]}" % a['x']).format(c)

Upvotes: 1

Joe T. Boka
Joe T. Boka

Reputation: 6581

Python2

If you want to print this is a testing, this is one way to do it:

a={'x':'test'}
b="this is a %sing" %(a['x'])
print b

Output:

this is a testing

Upvotes: -1

Vivek Sable
Vivek Sable

Reputation: 10223

Need to create one more dictionary.

>>> a={'x':'test'}
>>> b="this is a %s"
>>> c = {'test':'testing'}
>>> print b % c[a['x']]
this is a testing

Upvotes: 1

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