I just learned there are truthy and falsy values in python which are different from the normal True and False.
Can someone please explain in depth what truthy and falsy values are? Where should I use them? What is the difference between truthy and True values and falsy and False values?
We use "truthy" and "falsy" to differentiate from the bool values True and False. A "truthy" value will satisfy the check performed by if or while statements. As explained in the documentation, all values are considered "truthy" except for the following, which are "falsy":
None
False
Numbers that are numerically equal to zero, including:
0
0.0
0j
decimal.Decimal(0)
fraction.Fraction(0, 1)
Empty sequences and collections, including:
[] - an empty list
{} - an empty dict
() - an empty tuple
set() - an empty set
'' - an empty str
b'' - an empty bytes
bytearray(b'') - an empty bytearray
memoryview(b'') - an empty memoryview
an empty range, like range(0)
objects for which
obj.__bool__() returns False
obj.__len__() returns 0, given that obj.__bool__ is undefined
As the comments described, it just refers to values which are evaluated to True or False.
For instance, to see if a list is not empty, instead of checking like this:
if len(my_list) != 0:
print("Not empty!")
You can simply do this:
if my_list:
print("Not empty!")
This is because some values, such as empty lists, are considered False when evaluated for a boolean value. Non-empty lists are True.
Similarly for the integer 0, the empty string "", and so on, for False, and non-zero integers, non-empty strings, and so on, for True.
The idea of terms like "truthy" and "falsy" simply refer to those values which are considered True in cases like those described above, and those which are considered False.
For example, an empty list ([]) is considered "falsy", and a non-empty list (for example, [1]) is considered "truthy".
See also this section of the documentation.
Python determines the truthiness by applying bool() to the type, which returns True or False which is used in an expression like if or while.
Here is an example for a custom class Vector2dand it's instance returning False when the magnitude (lenght of a vector) is 0, otherwise True.
import math
class Vector2d(object):
def __init__(self, x, y):
self.x = float(x)
self.y = float(y)
def __abs__(self):
return math.hypot(self.x, self.y)
def __bool__(self):
return bool(abs(self))
a = Vector2d(0,0)
print(bool(a)) #False
b = Vector2d(10,0)
print(bool(b)) #True
Note: If we wouldn't have defined __bool__ it would always return True, as instances of a user-defined class are considered truthy by default.
Example from the book: "Fluent in Python, clear, concise and effective programming"
Truthy values refer to the objects used in a boolean context and not so much the boolean value that returns true or false.Take these as an example:
>>> bool([])
False
>>> bool([1])
True
>>> bool('')
False
>>> bool('hello')
True
Where should you use Truthy or Falsy values ?
These are syntactic sugar, so you can always avoid them, but using them can make your code more readable and make you more efficient.
Moreover, you will find them in many code examples, whether in python or not, because it is considered good practice.
As mentioned in the other answers, you can use them in if tests and while loops. Here are two other examples in python 3 with default values combined with or, s being a string variable. You will extend to similar situations as well.
Without truthy
if len(s) > 0:
print(s)
else:
print('Default value')
with truthy it is more concise:
print(s or 'Default value')
In python 3.8, we can take advantage of the assignment expression :=
without truthy
if len(s) == 0:
s = 'Default value'
do_something(s)
with truthy it is shorter too
s or (s := 'Default value')
do_something(s)
or even shorter,
do_something(s or (s := 'Default value'))
Without the assignment expression, one can do
s = s or 'Default value'
do_something(s)
but not shorter. Some people find the s =... line unsatisfactory because it corresponds to
if len(s)>0:
s = s # HERE is an extra useless assignment
else:
s = "Default value"
nevertheless you can adhere to this coding style if you feel comfortable with it.
Any object in Python can be tested for its truth value. It can be used in an if or while condition or as operand of the Boolean operations.
The following values are considered False:
None
False
zero of any numeric type, for example, 0, 0L, 0.0, 0j.
any empty sequence, for example, '', (), [].
any empty mapping, for example, {}.
instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False.
All other values are considered True -- thus objects of many types are always true.
Operations and built-in functions that have a Boolean result always return 0 or False for false and 1 or True for true, unless otherwise stated.
In case of if (!id) {}
!expr returns false if its single operand can be converted to true; otherwise, returns true.
If a value can be converted to true, the value is so-called truthy. If a value can be converted to false, the value is so-called falsy.
Examples of expressions that can be converted to false are:
null;
NaN;
0;
empty string ("" or '' or ``);
undefined.
Even though the ! operator can be used with operands that are not Boolean values, it can still be considered a boolean operator since its return value can always be converted to a boolean primitive. To explicitly convert its return value (or any expression in general) to the corresponding boolean value, use a double NOT operator or the Boolean constructor.
Example:
n1 = !null // !t returns true
n2 = !NaN // !f returns true
n3 = !'' // !f returns true
n4 = !'Cat' // !t returns false
While in case of if (id != null) {} it will only check if the value in id is not equal to null
reference https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Logical_NOT
Falsy means something empty like empty list,tuple, as any datatype having empty values or None.
Truthy means :
Except are Truthy
Related
I was encountering a lot of bugs in my code because I expected this expression:
Boolean([]); to evaluate to false.
But this wasn't the case as it evaluated to true.
Therefore, functions that possibly returned [] like this:
// Where myCollection possibly returned [ obj1, obj2, obj3] or []
if(myCollection)
{
// ...
}else
{
// ...
}
did not do expected things.
Am I mistaken in assuming that [] an empty array?
Also, Is this behavior consistent in all browsers? Or are there any gotchas there too? I observed this behavior in Goolgle Chrome by the way.
From http://www.sitepoint.com/javascript-truthy-falsy/
The following values are always falsy:
false
0 (zero)
0n (BigInt zero)
"" (empty string)
null
undefined
NaN (a special Number value meaning Not-a-Number!)
All other values are truthy, including "0" (zero in quotes), "false" (false in quotes), empty functions, empty arrays ([]), and empty objects ({}).
Regarding why this is so, I suspect it's because JavaScript arrays are just a particular type of object. Treating arrays specially would require extra overhead to test Array.isArray(). Also, it would probably be confusing if true arrays behaved differently from other array-like objects in this context, while making all array-like objects behave the same would be even more expensive.
You should be checking the .length of that array to see if it contains any elements.
if (myCollection) // always true
if (myCollection.length) // always true when array has elements
if (myCollection.length === 0) // same as is_empty(myCollection)
While [] equals false, it evaluates to true.
yes, this sounds bad or at least a bit confusing. Take a look at this:
const arr = [];
if (arr) console.log("[] is truethy");
if (arr == false) console.log("however, [] == false");
In practice, if you want to check if something is empty,
then check the length. (The ?. operator makes sure that also null is covered.)
const arr = []; // or null;
if (!arr?.length) console.log("empty or null")
[]==false // returns true
This evaluates to true, because of the Abstract Equality Algorithm as mentioned here in the ECMA Specification #Section 11.9.3
If you run through algorithm, mentioned above.
In first iteration, the condition satisfied is,
Step 7: If Type(y) is Boolean, return the result of the comparison x == ToNumber(y).
Hence the above condition transforms to -> [] == 0
Now in second iteration, the condition satisfied on [] == 0:
Step 9: If Type(x) is Object and Type(y) is either String or Number, return the result of the comparison ToPrimitive(x) == y.
[] is an object, henceforth, on converting to primitive, it transforms to an empty string ''
Hence, the above condition transforms to -> '' == 0
In third iteration, condition satisfied, is:
Step 5: If Type(x) is String and Type(y) is Number, return the result of the comparison ToNumber(x) == y.
As we know, empty string, '' is a falsy value, hence transforming an empty string to number will return us a value 0.
Henceforth, our condition, will transform to -> 0 == 0
In fourth iteration, the first condition is satisfied, where the types are equal and the numbers are equal.
Henceforth, the final value of the [] == false reduces to 0 == 0 which is true.
Hope this answers your question. Otherwise, you can also refer this youtube video
I suspect it has something to do with discrete math and the way a conditional (if then) works. A conditional has two parts, but in the instance where the first part doesn't exist, regardless of the second part, it returns something called a vacuous truth.
Here is the example stated in the wikipedia page for vacuous truths
"The statement "all cell phones in the room are turned off" will be true when no cell phones are in the room. In this case, the statement "all cell phones in the room are turned on" would also be vacuously true"
The wikipedia even makes specific mention of JavaScript a little later.
https://en.wikipedia.org/wiki/Vacuous_truth#:~:text=In%20mathematics%20and%20logic%2C%20a,the%20antecedent%20cannot%20be%20satisfied.&text=One%20example%20of%20such%20a,Eiffel%20Tower%20is%20in%20Bolivia%22.
Also want to add, that all objects in JavaScript (arrays are objects too) are stored in memory as links and these links are always not null or zero, that's why Boolean({}) === true, Boolean([]) === true.
Also this is the reason why same objects (copied by value not the link) are always not equal.
{} == {} // false
let a = {};
let b = a; // copying by link
b == a // true
As in JavaScript, everything is an object so for falsy and empty, I use the below condition:
if(value && Object.keys(value).length){
// Not falsy and not empty
}
else{
// falsy or empty array/object
}
Let me start off by saying I understand the difference between =, ==, and
===. The first is used to assign the right-hand value to the left-hand variable, the second is used to compare the equivalency of the two values, and the third is used not just for equivalency but type comparison as well (ie true === 1 would return false).
So I know that almost any time you see if (... = ...), there's a pretty good chance the author meant to use ==.
That said, I don't entirely understand what's happening with these scripts:
var a = 5;
if (a = 6)
console.log("doop");
if (true == 2)
console.log('doop');
According to this Javascript type equivalency table, true is equivalent to 1 but not 0 or -1. Therefore it makes sense to me that that second script does not output anything (at least, it isn't in my Chrome v58.0.3029.110).
So why does the first script output to the console but the second doesn't? What is being evaluated by the first script's if statement?
I dug into my C# knowledge to help me understand, but in C# you cannot compile if (a = 5) Console.WriteLine("doop"); so I had to explicitly cast it to a bool by doing if (Convert.ToBoolean(a = 5)) but then that makes sense it would evaluate to true because according to MSDN's documentation, Convert.ToBool returns true if the value supplied is anything other than 0. So this didn't help me very much, because in JS only 1 and true are equal.
There's a difference between making an abstract equality comparison with == and performing a simple type cast to boolean from a number value. In a == comparison between a boolean and a number, the boolean value is converted to 0 or 1 before the comparison. Thus in
if (true == 2)
the value true is first converted to 1 and then compared to 2.
In a type cast situation like
if (x = 2)
the number is converted to boolean such that any non-zero value is true. That is, the value 2 is assigned to x and the value of the overall expression is 2. That is then tested as boolean as part of the evaluation of the if statement, and so is converted as true, since 2 is not 0.
The various values that evaluate to boolean false are 0, NaN, "", null, undefined, and of course false. Any other value is true when tested as a boolean (for example in an if expression).
Why does an assignment in an if statement equate to true?
It doesn't. An assignment is evaluated as whatever value is assigned.
This expression is a true value:
a = true
But this expression is a false value:
b = false
That's true whether or not you put it in an if statement or not.
if(true){
alert("hello");
}
if("string"){
alert("hello");
}
The code above shows that a string can be used as a substitute for Boolean value "true". Why does this work and does this mean that Boolean value "true" and string have the same data type?
A string as a statement, represents it's existence, in other words, if the string is set, it will return true, as if its "" it will return as false
This way you can check a variable if it is set or not
JavaScript's conditional operators such as if-else and a ? b : c don't require the condition to be of actual Boolean type but rather check whether it is truthy or a falsy value in JavaScript terms.
For example, such values as null and 0 are considered falsy so they would be treated like false in if-else and ternary operators.
In a broader sense this might be considered as a case of type coercion, see for example this question for more information. So when an interpreter sees non-boolean expression inside an if it is "re-writing" it like if (Boolean("string")) { ... }, i.e. invoking conversion from the expression to the exact boolean type.
if("string"){
alert("hello");
}
since you did not include an operator, as default, it just checks for the existence of the string, "string" == true.
JavaScript has a boolean type, with possible values true and false (both of which are keywords.) Any value can be converted to a boolean according to the following rules:
false, 0, empty strings (""), NaN, null, and undefined all become false.
All other values become true.
You can perform this conversion explicitly using the Boolean() function:
Boolean(""); // false
Boolean(234); // true
https://developer.mozilla.org/en-US/docs/Web/JavaScript/A_re-introduction_to_JavaScript
It may be that I'm just very groggy this morning, but I'm having trouble understanding why this returns as true:
_.some([null, 0, 'yes', false]); // true
I know that _.some() returns true if at least one of the elements passes the predicate test as true. But from my understanding, if no predicate is provided, _.identity() is used. But console.log-ing each of those elements individually with _.identity() didn't return true for any of them. So why does it return true?
Without a predicate, some uses identity, which uses the value itself, and 'yes' is truthy.
A quick dive through the annotated source (paying special attention to cb and the handling of missing predicates there) leaves you with, essentially, a coercion to boolean when they do:
if (predicate(obj[currentKey], currentKey, obj)) return true;
No predicate means you're working with the original value there, so if ('yes'), which is true.
You're not seeing true in the console for any of those values because _.identity will return the value itself (so 'yes') rather than coercing it to a boolean. If you were to do !!'yes' (coercion and double-not), you will see true.
'yes' is truthy:
_.some([null]) // false
_.some([0]) // false
_.some(['yes']) // true
_.some([false]) // false
From the Truth, equality in javascript link:
The construct if ( Expression ) Statement will coerce the result of evaluating the Expression to a boolean using the abstract method ToBoolean for which the ES5 spec defines the following algorithm:
string: The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
It doesn't need to return the literal value true, it only needs to return a truthy value (although you always should return only booleans).
The non-empty string 'yes' is truthy (you can test by Boolean('yes') or !!'yes').
This question already has answers here:
What is the !! (not not) operator in JavaScript?
(42 answers)
Closed 8 years ago.
I am by no means an expert at Javascript, but I have been reading Mark Pilgrim's "Dive into HTML5" webpage and he mentioned something that I would like a better understanding of.
He states:
Finally, you use the double-negative trick to force the result to a Boolean value (true or false).
function supports_canvas() {
return !!document.createElement('canvas').getContext;
}
If anyone can explain this a little better I would appreciate it!
A logical NOT operator ! converts a value to a boolean that is the opposite of its logical value.
The second ! converts the previous boolean result back to the boolean representation of its original logical value.
From these docs for the Logical NOT operator:
Returns false if its single operand can be converted to true; otherwise, returns true.
So if getContext gives you a "falsey" value, the !! will make it return the boolean value false. Otherwise it will return true.
The "falsey" values are:
false
NaN
undefined
null
"" (empty string)
0
Javascript has a confusing set of rules for what is considered "true" and "false" when placed in a context where a Boolean is expected. But the logical-NOT operator, !, always produces a proper Boolean value (one of the constants true and false). By chaining two of them, the idiom !!expression produces a proper Boolean with the same truthiness as the original expression.
Why would you bother? Because it makes functions like the one you show more predictable. If it didn't have the double negative in there, it might return undefined, a Function object, or something not entirely unlike a Function object. If the caller of this function does something weird with the return value, the overall code might misbehave ("weird" here means "anything but an operation that enforces Boolean context"). The double-negative idiom prevents this.
In javascript, using the "bang" operator (!) will return true if the given value is true, 1, not null, etc. It will return false if the value is undefined, null, 0, or an empty string.
So the bang operator will always return a boolean value, but it will represent the opposite value of what you began with. If you take the result of that operation and "bang" it again, you can reverse it again, but still end up with a boolean (and not undefined, null, etc).
Using the bang twice will take a value that could have been undefined, null, etc, and make it just plain false. It will take a value that could have been 1, "true", etc. and make it just plain true.
The code could have been written:
var context = document.createElement('canvas').getContext;
var contextDoesNotExist = !context;
var contextExists = !contextDoesNotExist;
return contextExists;
Using !!variable gives you a guarantee of typecast to boolean.
To give you a simple example:
"" == false (is true)
"" === false (is false)
!!"" == false (is true)
!!"" === false (is true)
But it doesn't make sense to use if you are doing something like:
var a = ""; // or a = null; or a = undefined ...
if(!!a){
...
The if will cast it to boolean so there is no need to make the implicit double negative cast.
! casts "something"/"anything" to a boolean.
!! gives the original boolean value back (and guarantees the expression is a boolean now, regardless to what is was before)
The first ! coerces the variable to a boolean type and inverts it. The second ! inverts it again (giving you the original (correct) boolean value for whatever you are checking).
For clarity you would be better off using
return Boolean(....);
document.createElement('canvas').getContext may evaluate to either undefined or an object reference. !undefined yields true, ![some_object] yields false. This is almost what we need, just inverted. So !! serves to convert undefined to false and an object reference to true.
It's to do with JavaScript's weak typing. document.createElement('canvas').getContext is a function object. By prepending a single ! it evaluates it as a boolean expression and flips the answer around. By prepending another !, it flips the answer back. The end result is that the function evaluates it as a boolean expression, but returns an actual boolean result rather than the function object itself. Prepending !! is a quick and dirty way to typecast an expression to a boolean type.
If document.createElement('canvas').getContext isn't undefined or null, it will return true. Otherwise it will return false.