Empty strings are "falsy" (python 2 or python 3 reference), which means they are considered false in a Boolean context, so you can just do this:
if not myString:
This is the preferred way if you know that your variable is a string. If your variable could also be some other type then you should use myString == "". See the documentation on Truth Value Testing for other values that are false in Boolean contexts.
From PEP 8, in the “Programming Recommendations” section:
For sequences, (strings, lists, tuples), use the fact that empty sequences are false.
So you should use:
if not some_string:
or:
if some_string:
Just to clarify, sequences are evaluated to False or True in a Boolean context if they are empty or not. They are not equal to False or True.
The most elegant way would probably be to simply check if its true or falsy, e.g.:
if not my_string:
However, you may want to strip white space because:
>>> bool("")
False
>>> bool(" ")
True
>>> bool(" ".strip())
False
You should probably be a bit more explicit in this however, unless you know for sure that this string has passed some kind of validation and is a string that can be tested this way.
I would test noneness before stripping. Also, I would use the fact that empty strings are False (or Falsy). This approach is similar to Apache's StringUtils.isBlank or Guava's Strings.isNullOrEmpty
This is what I would use to test if a string is either None OR Empty OR Blank:
def isBlank (myString):
if myString and myString.strip():
#myString is not None AND myString is not empty or blank
return False
#myString is None OR myString is empty or blank
return True
And, the exact opposite to test if a string is not None NOR Empty NOR Blank:
def isNotBlank (myString):
if myString and myString.strip():
#myString is not None AND myString is not empty or blank
return True
#myString is None OR myString is empty or blank
return False
More concise forms of the above code:
def isBlank (myString):
return not (myString and myString.strip())
def isNotBlank (myString):
return bool(myString and myString.strip())
The only really solid way of doing this is the following:
if "".__eq__(myString):
All other solutions have possible problems and edge cases where the check can fail.
len(myString)==0 can fail if myString is an object of a class that inherits from str and overrides the __len__() method.
Similarly myString == "" and myString.__eq__("") can fail if myString overrides __eq__() and __ne__().
For some reason "" == myString also gets fooled if myString overrides __eq__().
myString is "" and "" is myString are equivalent. They will both fail if myString is not actually a string but a subclass of string (both will return False). Also, since they are identity checks, the only reason why they work is because Python uses String Pooling (also called String Internment) which uses the same instance of a string if it is interned (see here: Why does comparing strings using either '==' or 'is' sometimes produce a different result?). And "" is interned from the start in CPython
The big problem with the identity check is that String Internment is (as far as I could find) that it is not standardised which strings are interned. That means, theoretically "" is not necessary interned and that is implementation dependant.
The only way of doing this that really cannot be fooled is the one mentioned in the beginning: "".__eq__(myString). Since this explicitly calls the __eq__() method of the empty string it cannot be fooled by overriding any methods in myString and solidly works with subclasses of str.
Also relying on the falsyness of a string might not work if the object overrides it's __bool__() method.
This is not only theoretical work but might actually be relevant in real usage since I have seen frameworks and libraries subclassing str before and using myString is "" might return a wrong output there.
Also, comparing strings using is in general is a pretty evil trap since it will work correctly sometimes, but not at other times, since string pooling follows pretty strange rules.
That said, in most cases all of the mentioned solutions will work correctly. This is post is mostly academic work.
I find hardcoding(sic) "" every time for checking an empty string not as good.
Doing this: foo == "" is very bad practice. "" is a magical value. You should never check against magical values (more commonly known as magical numbers)
What you should do is compare to a descriptive variable name.
One may think that "empty_string" is a descriptive variable name. It isn't.
Before you go and do empty_string = "" and think you have a great variable name to compare to. This is not what "descriptive variable name" means.
A good descriptive variable name is based on its context. You have to think about what the empty string is.
You are building a form where a user can enter values. You want to check if the user wrote something or not.
A good variable name may be not_filled_in
This makes the code very readable
if formfields.name == not_filled_in:
raise ValueError("We need your name")
You are parsing CSV files and want the empty string to be parsed as None
(Since CSV is entirely text based, it cannot represent None without using predefined keywords)
A good variable name may be CSV_NONE
This makes the code easy to change and adapt if you have a new CSV file that represents None with another string than ""
if csvfield == CSV_NONE:
csvfield = None
There are no questions about if this piece of code is correct. It is pretty clear that it does what it should do.
Compare this to
if csvfield == EMPTY_STRING:
csvfield = None
The first question here is, Why does the empty string deserve special treatment?
This would tell future coders that an empty string should always be considered as None.
This is because it mixes business logic (What CSV value should be None) with code implementation (What are we actually comparing to)
There needs to be a separation of concern between the two.
Responding to @1290. Sorry, no way to format blocks in comments. The None value is not an empty string in Python, and neither is (spaces). The answer from Andrew Clark is the correct one: if not myString. The answer from @rouble is application-specific and does not answer the OP's question. You will get in trouble if you adopt a peculiar definition of what is a "blank" string. In particular, the standard behavior is that str(None) produces 'None', a non-blank string.
However if you must treat None and (spaces) as "blank" strings, here is a better way:
class weirdstr(str):
def __new__(cls, content):
return str.__new__(cls, content if content is not None else '')
def __nonzero__(self):
return bool(self.strip())
Examples:
>>> normal = weirdstr('word')
>>> print normal, bool(normal)
word True
>>> spaces = weirdstr(' ')
>>> print spaces, bool(spaces)
False
>>> blank = weirdstr('')
>>> print blank, bool(blank)
False
>>> none = weirdstr(None)
>>> print none, bool(none)
False
>>> if not spaces:
... print 'This is a so-called blank string'
...
This is a so-called blank string
Meets the @rouble requirements while not breaking the expected bool behavior of strings.
I did some experimentation with strings like '', ' ', '\n', etc. I want isNotWhitespace to be True if and only if the variable foo is a string with at least one non-whitespace character. I'm using Python 3.6. Here's what I ended up with:
isWhitespace = str is type(foo) and not foo.strip()
isNotWhitespace = str is type(foo) and not not foo.strip()
Wrap this in a method definition if desired.
You may have a look at this Assigning empty value or string in Python
This is about comparing strings that are empty. So instead of testing for emptiness with not, you may test is your string is equal to empty string with "" the empty string...
for those who expect a behaviour like the apache StringUtils.isBlank or Guava Strings.isNullOrEmpty :
if mystring and mystring.strip():
print "not blank string"
else:
print "blank string"
If you just use
not var1
it is not possible to difference a variable which is boolean False from an empty string '':
var1 = ''
not var1
> True
var1 = False
not var1
> True
However, if you add a simple condition to your script, the difference is made:
var1 = False
not var1 and var1 != ''
> True
var1 = ''
not var1 and var1 != ''
> False
In case this is useful to someone, here is a quick function i built out to replace blank strings with N/A's in lists of lists (python 2).
y = [["1","2",""],["1","4",""]]
def replace_blank_strings_in_lists_of_lists(list_of_lists):
new_list = []
for one_list in list_of_lists:
new_one_list = []
for element in one_list:
if element:
new_one_list.append(element)
else:
new_one_list.append("N/A")
new_list.append(new_one_list)
return new_list
x= replace_blank_strings_in_lists_of_lists(y)
print x
This is useful for posting lists of lists to a mysql database that does not accept blanks for certain fields (fields marked as NN in schema. in my case, this was due to a composite primary key).
""not as good? – NoName