How to check if the string is empty

Question

Does Python have something like an empty string variable where you can do   if myString    string empty    Regardless  what s the most elegant way to check for empty string values  I find hard coding    every time for checking an empty string not as good

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Test empty or blank string  shorter way    if myString strip        print  it s not an empty or blank string   else      print  it s an empty or blank string

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str      if not str     print  Empty String  if len str   0      print  Empty String

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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   39     39  or   39 is  39  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

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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

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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  gt  True  var1   False not var1  gt  True   However  if you add a simple condition to your script  the difference is made   var1    False not var1 and var1        gt  True  var1      not var1 and var1        gt  False

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As prmatta posted above  but with mistake   def isNoneOrEmptyOrBlankString  myString       if myString          if not myString strip                return True         else              return False     return False

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If you want to differentiate between empty and null strings  I would suggest using if len string   otherwise  I d suggest using simply if string as others have said   The caveat about strings full of whitespace still applies though  so don t forget to strip

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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

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I find this elegant as it makes sure it is a string and checks its length   def empty mystring       assert isinstance mystring  str      if len mystring     0          return True     else          return False

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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

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When you are reading file by lines and want to determine  which line is empty  make sure you will use  strip    because there is new line character in  empty  line   lines   open  my file log    r   readlines    for line in lines      if not line strip            continue        your code for non-empty lines

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not str myString    This expression is True for strings that are empty  Non-empty strings  None and non-string objects will all produce False  with the caveat that objects may override   str   to thwart this logic by returning a falsy value

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Empty strings are  falsy  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

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I find hardcoding sic   quot  quot  every time for checking an empty string not as good   Clean code approach Doing this  foo     quot  quot  is very bad practice   quot  quot  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  Descriptive variable names One may think that  quot empty string quot  is a descriptive variable name  It isn t  Before you go and do empty string    quot  quot  and think you have a great variable name to compare to  This is not what  quot descriptive variable name quot  means  A good descriptive variable name is based on its context  You have to think about what the empty string is   Where does it come from  Why is it there  Why do you need to check for it   Simple form field example 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  quot We need your name quot    Thorough CSV parsing example 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  quot  quot  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

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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

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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     gt  gt  gt  bool      False   gt  gt  gt  bool         True   gt  gt  gt  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

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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

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Another easy way could be to define a simple function     def isStringEmpty inputString       if len inputString     0          return True     else          return False

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a      b         a isspace   - gt  False b isspace   - gt  True

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How about this   Perhaps it s not  the most elegant   but it seems pretty complete and clear   if  s is None  or  str s  strip            STRING s IS  EMPTY

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Below is a simple method to identify an empty string  a  quot  quot   Empty string if len a   0    len   is used to find the length of the string or variable inside its  brackets     print  quot Empty quot   else     print  quot Filled quot    The above len   method is an inbuilt function that helps us to find the length of a certain string or variable  It can also be used in the case of lists   LOGIC  If the length of the character or variable is 0 then absolutely it means that the string or variable is empty  BUT remember that this will work only in the case of strings and lists and not with integers  Below is the code to find length of integers   a int input    if len str a    0      print True   In the above code str   is used to convert an int to a string format like from 123 to its string format  quot 123 quot   In the string format it will be valid

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if stringname  gives a false when the string is empty  I guess it can t be simpler than this

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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    gt  gt  gt  normal   weirdstr  word    gt  gt  gt  print normal  bool normal  word True   gt  gt  gt  spaces   weirdstr         gt  gt  gt  print spaces  bool spaces      False   gt  gt  gt  blank   weirdstr      gt  gt  gt  print blank  bool blank   False   gt  gt  gt  none   weirdstr None   gt  gt  gt  print none  bool none   False   gt  gt  gt  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

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I once wrote something similar to Bartek s answer and javascript inspired   def is not blank s       return bool s and s strip      Test   print is not blank          False print is not blank          False print is not blank  ok      True print is not blank None     False

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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

[python] How to check if the string is empty?

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