Ok...so, it's kind of easy.

The old method:

num = 6
mystr = 'number is %s' % num
print(mystr) # number is 6

The newer .format method:

num = 6
mystr = "number is {}".format(num)
print(mystr) # number is 6

The .format method using named variable (useful for when sequence can't be depended upon):

num = 6
mystr = "number is {num}".format(num=num)
print(mystr) # number is 6

The shorter f-string method: (thank you @mayur)

num = 6
mystr = f"number is {num}"
print(mystr) # number is 6

Python 3.6+ does have variable interpolation - prepend an f to your string:

f"foo is {bar}"

For versions of Python below this (Python 2 - 3.5) you can use str.format to pass in variables:

# Rather than this:
print("foo is #{bar}")

# You would do this:
print("foo is {}".format(bar))

# Or this:
print("foo is {bar}".format(bar=bar))

# Or this:
print("foo is %s" % (bar, ))

# Or even this:
print("foo is %(bar)s" % {"bar": bar})

Modules don't share namespaces in python, so globals() for my_print is always going to be the globals() of my_print.py file ; i.e the location where the function was actually defined.

def mprint(string='', dic = None):
    dictionary = dic if dic is not None else globals()
    print string.format(**dictionary)

You should pass the current module's globals() explicitly to make it work.

Ans don't use mutable objects as default values in python functions, it can result in unexpected results. Use None as default value instead.

A simple example for understanding scopes in modules:

file : my_print.py

x = 10
def func():
    global x
    x += 1
    print x

file : main.py

from my_print import *
x = 50
func()   #prints 11 because for func() global scope is still 
         #the global scope of my_print file
print x  #prints 50

Python 3.6 will add literal string interpolation similar to Ruby's string interpolation. Starting with that version of Python (which is scheduled to be released by the end of 2016), you will be able to include expressions in "f-strings", e.g.

name = "Spongebob Squarepants"
print(f"Who lives in a Pineapple under the sea? {name}.")

Prior to 3.6, the closest you can get to this is

name = "Spongebob Squarepants"
print("Who lives in a Pineapple under the sea? %(name)s." % locals())

The % operator can be used for string interpolation in Python. The first operand is the string to be interpolated, the second can have different types including a "mapping", mapping field names to the values to be interpolated. Here I used the dictionary of local variables locals() to map the field name name to its value as a local variable.

The same code using the .format() method of recent Python versions would look like this:

name = "Spongebob Squarepants"
print("Who lives in a Pineapple under the sea? {name!s}.".format(**locals()))

There is also the string.Template class:

tmpl = string.Template("Who lives in a Pineapple under the sea? $name.")
print(tmpl.substitute(name="Spongebob Squarepants"))