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

user_116 0
Viewed 265964

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)
idioms python string
16 Answers
def xstr(s):
    return '' if s is None else str(s)
user_128550

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)
user_430890

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) # -> ''
user_1189030

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

user_958100 
def xstr(s):
   return s or ""
user_452260

Functional way (one-liner)

xstr = lambda s: '' if s is None else s
user_1054590 
def xstr(s):
    return {None:''}.get(s, s)
user_1222940

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

xstr = lambda s: s is not None and s or ''
user_1209710

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)
user_11610250 
def xstr(s):
    return s if s else ''

s = "%s%s" % (xstr(a), xstr(b))
user_933800

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 '')
user_1165400

Use F string if you are using python v3.7

xstr = F"{s}"
user_17090880
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