Python: most idiomatic way to convert None to empty string?

Probably the shortest would be str(s or '')

Because None is False, and "x or y" returns y if x is false. See Boolean Operators for a detailed explanation. It's short, but not very explicit.

def xstr(s):
    return '' if s is None else str(s)

If you actually want your function to behave like the str() built-in, but return an empty string when the argument is None, do this:

def xstr(s):
    if s is None:
        return ''
    return str(s)

If you know that the value will always either be a string or None:

xstr = lambda s: s or ""

print xstr("a") + xstr("b") # -> 'ab'
print xstr("a") + xstr(None) # -> 'a'
print xstr(None) + xstr("b") # -> 'b'
print xstr(None) + xstr(None) # -> ''

return s or '' will work just fine for your stated problem!

def xstr(s):
   return s or ""

Functional way (one-liner)

xstr = lambda s: '' if s is None else s

A neat one-liner to do this building on some of the other answers:

s = (lambda v: v or '')(a) + (lambda v: v or '')(b)

or even just:

s = (a or '') + (b or '')

UPDATE:

I mainly use this method now:

some_string = None
some_string or ''

If some_string was not NoneType, the or would short circuit there and return it, otherwise it returns the empty string.

OLD:

Max function worked in python 2.x but not in 3.x:

max(None, '')  # Returns blank
max("Hello", '') # Returns Hello

def xstr(s):
    return {None:''}.get(s, s)

Variation on the above if you need to be compatible with Python 2.4

xstr = lambda s: s is not None and s or ''

If it is about formatting strings, you can do the following:

from string import Formatter

class NoneAsEmptyFormatter(Formatter):
    def get_value(self, key, args, kwargs):
        v = super().get_value(key, args, kwargs)
        return '' if v is None else v

fmt = NoneAsEmptyFormatter()
s = fmt.format('{}{}', a, b)

def xstr(s):
    return s if s else ''

s = "%s%s" % (xstr(a), xstr(b))

We can always avoid type casting in scenarios explained below.

customer = "John"
name = str(customer)
if name is None
   print "Name is blank"
else: 
   print "Customer name : " + name

In the example above in case variable customer's value is None the it further gets casting while getting assigned to 'name'. The comparison in 'if' clause will always fail.

customer = "John" # even though its None still it will work properly.
name = customer
if name is None
   print "Name is blank"
else: 
   print "Customer name : " + str(name)

Above example will work properly. Such scenarios are very common when values are being fetched from URL, JSON or XML or even values need further type casting for any manipulation.

Use short circuit evaluation:

s = a or '' + b or ''

Since + is not a very good operation on strings, better use format strings:

s = "%s%s" % (a or '', b or '')

Use F string if you are using python v3.7

xstr = F"{s}"