Strings with embedded variables in Python

Question

Maybe some already asked this but I didn't find it and I wanted to know how to embed variables into a string in Python. I usually do it like this:

print('Hi, my name is %s and my age is %d' %(name, age))

But sometimes is confusing and with ruby it would be like this

puts('Hi, my name is #{name} and my age is #{age}')

Is there any way to do in Python like I to it in Ruby?

Answer 1

From Python 3.6 onwards, you can use an Formatting string literal (aka f-strings), which takes any valid Python expression inside {...} curly braces, followed by optional formatting instructions:

print(f'Hi, my name is {name} and my age is {age:d}')

Here name and age are both simple expressions that produce the value for that name.

In versions preceding Python 3.6, you can use str.format(), paired with either locals() or globals():

print('Hi, my name is {name} and my age is {age}'.format(**locals()))

As you can see the format is rather close to Ruby's. The locals() and globals() methods return namespaces as a dictionary, and the ** keyword argument splash syntax make it possible for the str.format() call to access all names in the given namespace.

Demo:

>>> name = 'Martijn'
>>> age = 40
>>> print('Hi, my name is {name} and my age is {age}'.format(**locals()))
Hi, my name is Martijn and my age is 40

Note however that explicit is better than implicit and you should really pass in name and age as arguments:

print('Hi, my name is {name} and my age is {age}'.format(name=name, age=age)

or use positional arguments:

print('Hi, my name is {} and my age is {}'.format(name, age))

Answer 2

You can also use:

dicta = {'hehe' : 'hihi', 'haha': 'foo'}
print 'Yo %(hehe)s %(haha)s' % dicta

Answer 3

An alternative way that uses exactly Ruby's syntax for the format string:

import string
class RubyTemplate(string.Template):
    delimiter = '#'

Used as:

>>> t = RubyTemplate('Hi, my name is #{name} and my age is #{age}')
>>> name = 'John Doe'
>>> age = 42
>>> t.substitute(**locals())
'Hi, my name is John Doe and my age is 42'

You can then create a function such as:

def print_template(template, vars):
    print(RubyTemplate(template).substitute(**vars))

And use it as:

>>> print_template('Hi, my name is #{name} and my age is #{age}', locals())
Hi, my name is John Doe and my age is 42

---

On a side note: even python's % allow this kind of interpolation:

>>> 'Hi, my name is %(name)s and my age is %(age)d' % locals()
'Hi, my name is John Doe and my age is 42'

Answer 4

str.format is the new way to do it, but this also works:

print('Hi, my name is %(name)s and my age is %(age)d' % {
   'name': name,
   'age': age,
})

Which is really the same as this, if the variables already exist:

print('Hi, my name is %(name)s and my age is %(age)d' % locals())