Python how to check a list does not contain any value

Question

consider this simple function

def foo(l=[]): if not l: print "List is empty" else : print "List is not empty" 

Now let foo

 x=[] foo(x) #List is empty foo('') #List is empty 

But if x = [''] the list is not considered empty !!!

 x=[''] foo(x) #List is not empty 

Questions -

• Why is the list of empty values ​​not considered empty? (In the case of a variable, it is considered empty, for example)

   x='' if x:print 'not empty!!' else: print 'empty' 

• How to change the function foo () so that the list is considered empty in all these cases: x=[] , x=[''] , x=['', '']

Answer 1

Using the built-in any()

 def foo(l=[]): if any(l): print 'List is not empty' else: print 'List is empty' foo(['']) # List is empty 

Answer 2

In your examples, the only case where the list is really empty is the one in which there is nothing in square brackets. In other examples, you have lists with different numbers of blank lines. These are just different (and in all languages ​​that I can think of, the same would be true).

Answer 3

First of all: even an empty string is a string. The list containing the empty string still contains the item.

while a='' empty with len = 0, it is independent of the list, it still contains an element, for example, mylist = [a] matches mylist = [''] , but it may be more clear to you. Take a as an element and ignore the contents.

To check if list items are empty, iterate over them.

 def isEmpty(li=[]): for elem in li: if len(elem) > 0: return false return true 

Answer 4

You can use the recursive function call foo to work with nested lists.

 def foo(l=[]): if type(l)==list: return any([foo(x) for x in l]) else: return bool(l) 

Answer 5

To answer your first question about why the list of empty values ​​is not considered empty, it is because it contains something, even if these things are empty themselves. Think of it as a box with empty drawers.

The code below shows one way to modify the foo() function to do what you want (and test). Your idea of ​​what an empty list is was surprisingly difficult to develop, in part because it contradicts what the language itself considers empty. As you can see, all the logic involved in determining whether the list is “empty” according to your definition has been moved to a separate function called empty_list() , since this probably has little to do with the rest of what foo() has to achieve. It is not too difficult, and if nothing else gives you a good starting point.

Also, you did not say what it should do if the argument it passed is not a list of any type or was not a list, but does not contain only other lists or strings, therefore, as it is written, it will raise a TypeError exception - something similar to how most Python built-in functions react when it happens to them. The following is an example code and its test result:

 try: string_type = basestring except NameError: # probably Python 3.x string_type = str class _NULL(object): # unique marker object def __repr__(self): return '<nothing>' _NULL = _NULL() def empty_list(arg=_NULL): arg = arg if arg is not _NULL else [] if not isinstance(arg, (list, string_type)): raise TypeError elif isinstance(arg, string_type): return not len(arg) else: return len(arg) == 0 or all(empty_list(e) for e in arg) def foo(list_=None): if list_ is None or empty_list(list_): print 'list is empty' else: print 'list is not empty' testcases = [ _NULL, [], [''], ['', ''], ['', ['']], ['abc'], ['', 'abc'], [False], [None], [0], [0.0], [0L], [0j], [42], [{}], [{'':0}], [{'a':1}], False, None, 0, 0.0, 0L, 0j, 42, {}, {'':0}, {'a':1}, ] for arg in testcases: call = 'foo( {!r:s} ) ->'.format(arg) print '{!s:>20s}'.format(call), try: foo() if arg is _NULL else foo(arg) except TypeError: print 'illegal argument exception' 

Here is the output that it produces using Python 2.7:

  foo( <nothing> ) -> list is empty foo( [] ) -> list is empty foo( [''] ) -> list is empty foo( ['', ''] ) -> list is empty foo( ['', ['']] ) -> list is empty foo( ['abc'] ) -> list is not empty foo( ['', 'abc'] ) -> list is not empty foo( [False] ) -> illegal argument exception foo( [None] ) -> illegal argument exception foo( [0] ) -> illegal argument exception foo( [0.0] ) -> illegal argument exception foo( [0L] ) -> illegal argument exception foo( [0j] ) -> illegal argument exception foo( [42] ) -> illegal argument exception foo( [{}] ) -> illegal argument exception foo( [{'': 0}] ) -> illegal argument exception foo( [{'a': 1}] ) -> illegal argument exception foo( False ) -> illegal argument exception foo( None ) -> list is empty foo( 0 ) -> illegal argument exception foo( 0.0 ) -> illegal argument exception foo( 0L ) -> illegal argument exception foo( 0j ) -> illegal argument exception foo( 42 ) -> illegal argument exception foo( {} ) -> illegal argument exception foo( {'': 0} ) -> illegal argument exception foo( {'a': 1} ) -> illegal argument exception 

Answer 6

The list [''] is indeed not empty. It contains an empty string. The line is empty, the list is missing. If you want to find these lists, check to see if it is empty, and if not, check to see if there is every entry. ''