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

Question:

What is the most idiomatic way to do the following?

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

s = xstr(a) + xstr(b)

update: I’m incorporating Tryptich’s suggestion to use str(s), which makes this routine work for other types besides strings. I’m awfully impressed by Vinay Sajip’s lambda suggestion, but I want to keep my code relatively simple.

def xstr(s):
    if s is None:
        return ''
    else:
        return str(s)
Asked By: Mark Harrison

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Answers:

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

s = "%s%s" % (xstr(a), xstr(b))
Answered By: phillc
def xstr(s):
    return {None:''}.get(s, s)
Answered By: tobidope

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)
Answered By: Kenan Banks

Functional way (one-liner)

xstr = lambda s: '' if s is None else s
Answered By: Dario

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 '')
Answered By: sharjeel

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) # -> ''
Answered By: Vinay Sajip

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

Answered By: Alex Martelli

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

xstr = lambda s: s is not None and s or ''
Answered By: Peter Ericson
def xstr(s):
   return s or ""
Answered By: Krystian Cybulski

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.

Answered By: dorvak

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
Answered By: radtek

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.

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 '')
Answered By: Willem van Ketwich

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)
Answered By: maciek

Use F string if you are using python v3.7

xstr = F"{s}"
Answered By: guagay_wk
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