This question already has answers here:
You need parens:
The problem is the lexer thinks "4." is going to be a floating-point number.
Also, this works:
x = 4
Programming in Python 3: A Complete Introduction to the Python , In general, Python allows us to call methods or access attributes on any literal, data type provides the called method or the attribute—however, this does not apply to The functions in the math module do not work with complex numbers. 123 is just as much of an object as 3.14, the "problem" lies within the grammar rules of the language; the parser thinks we are about to define a float — not an int with a trailing method call. We will get the expected behavior if we wrap the number in parenthesis, as in the below.
So you think you can dance floating-point?
123 is just as much of an object as
3.14, the "problem" lies within the grammar rules of the language; the parser thinks we are about to define a
float — not an int with a trailing method call.
We will get the expected behavior if we wrap the number in parenthesis, as in the below.
Or if we simply add some whitespace after
>>> 123 .__str__()
The reason it does not work for
123.__str__() is that the
dot following the is interpreted as the
decimal-point of some partially declared floating-point. >>> 123.__str__()
File "", line 1
SyntaxError: invalid syntax
The parser tries to interpret
__str__() as a sequence of digits, but obviously fails — and we get a
SyntaxError basically saying that the parser stumbled upon something that it did not expect.
When looking at
123.__str__() the python parser could use either
3 characters and interpret these 3 characters as an integer, or it could use 4 characters and interpret these as the start of a floating-point. 123.__str__()
^^^ - int
^^^^- start of floating-point
Just as a little child would like as much cake as possible on their plate, the parser is greedy and would like to swallow as much as it can all at once — even if this isn't always the best of ideas —as such the latter ("better") alternative is chosen.
When it later realizes that
__str__() can in no way be interpreted as the
decimals of a floating-point it is already too late; SyntaxError.
Note 123 .__str__() # works fine
In the above snippet,
123 (note the space) must be interpreted as an
integer since no number can contain spaces. This means that it is semantically equivalent to
Note 123..__str__() # works fine
The above also works because a
number can contain at most one decimal-point, meaning that it is equivalent to
This section contains the lexical definition of the relevant literals.
Lexical analysis - 2.4.5 Floating point literals floatnumber ::= pointfloat | exponentfloat
pointfloat ::= [intpart] fraction | intpart "."
exponentfloat ::= (intpart | pointfloat) exponent
intpart ::= digit+
fraction ::= "." digit+
exponent ::= ("e" | "E") ["+" | "-"] digit+
Lexical analysis - 2.4.4 Integer literals integer ::= decimalinteger | octinteger | hexinteger | bininteger
decimalinteger ::= nonzerodigit digit* | "0"+
nonzerodigit ::= "1"..."9"
digit ::= "0"..."9"
octinteger ::= "0" ("o" | "O") octdigit+
hexinteger ::= "0" ("x" | "X") hexdigit+
bininteger ::= "0" ("b" | "B") bindigit+
octdigit ::= "0"..."7"
hexdigit ::= digit | "a"..."f" | "A"..."F"
bindigit ::= "0" | "1"
Reference Manual for the Ada Programming Language, The attributes defined by the language are summarized in Annex A. In addition, a universal expression are those for certain operations of array types (see 3.6.2). The literal null yields a null access value which designates no objects at all. Parameters to an attribute constructor are limited to simple types/literals: bool, int, double, string, Type, enums, etc and arrays of those types. You can not use an expression or a variable. You are free to use positional or named parameters. How to create your own attribute. Creating an attribute is as simple as inheriting from the Attribute
Add a space after the
Otherwise, the lexer will split this expression into the tokens
")", i.e. the first token is interpreted as a floating point number. The lexer always tries to build the longest possible token.
The SGML FAQ Book: Understanding the Foundation of HTML and XML, In attributes, SGML has several rules you should know unless your authoring systems completely hides them. It is not trivial for software to do this perfectly, so you may well run into period, and hyphen (element types, attribute names, and entity names are all Quoted attribute values are called “attribute value literals". // Here 100 is a constant/literal. int x = 100; Integral literals. For Integral data types (byte, short, int, long), we can specify literals in 4 ways:-Decimal literals (Base 10) : In this form the allowed digits are 0-9. int x = 101; Octal literals (Base 8) : In this form the allowed digits are 0-7. // The octal number should be prefix with 0.
actually (to increase unreadability...):
is valid, too. it gives
'0x1.0000000000000p+2' -- but then it's a float, of course...
Ada 95 Reference Manual. Language and Standard Libraries: , NOTES 13 4 Attributes are defined throughout this International Standard, and (or a range attribute reference) has to be resolved without using any context. However, in the case of the Access attribute, the expected type for the prefix has to 7 A literal null shall not be of an anonymous access type, since such types do not� The type of an integer literal is determined by its suffix as follows: If the literal has no suffix, its type is the first of the following types in which its value can be represented: int, uint, long, ulong. If the literal is suffixed by U or u, its type is the first of the following types in which its value can be represented: uint, ulong. Attribute-Based Encryption and Access Control, Thus, attributes are used to define users' access groups that provide group- based First, it is not always able to derive one or multiple attribute policy trees to one or multiple attribute policy trees in order to construct access policies for all attribute literals in conjunctive/disjunctive normal forms as an attribute policy tree� Accessing Attributes. You can access attributes by creating an object of the class, and by using the dot syntax (.): The following example will create an object of the MyClass class, with the name myObj. We use the x attribute on the object to print its value: Database and Expert Systems Applications: Proceedings of the , However, if Access(x, y) contains all constants (computer accounts in this case) appearing in the attribute domain of y in the second column, then there is no need Now consider an example with negative literals in the body V,Vy[A(x) — - B(x, y)]. contained in the transformed rule, we can dispense with queries and work� Coordinate actions of all concepts associated with a use case, a logical grouping of use cases, or the entire system and delegate the work to other concepts. - Controller Operate the alarm bell to signal possible break-ins. - AlarmOperator Verify whether or not the key-code entered by the user is valid. - KeyChecker
2. Working with Data: Literals, Values, Variables, and Types , Working with Data: Literals, Values, Variables, and Types In this chapter we will All data in Scala corresponds to a specific type, and all Scala types are Values require both a name and assigned data, but they do not require an explicit type. it will retain the same value regardless of any other code that may access it. Constants refer to fixed values that the program may not alter during its execution. These fixed values are also called literals. Constants can be of any of the basic data types like an integer constant, a floating constant, a character constant, or a string literal. There are enumeration constants as well.
Comments Look at what the message says:
invalid syntax. That is an indication that this has nothing to do with the meaning of the construct, just about how your wrote it down.
4. , Why is
4. a number in python?
@Abderrahimben: The lexers of most programming languages work by building the longest possible token. Cases with ambiguous tokenization are generally rare and should be avoided, and trying to automatically resolve the ambiguities would significantly complicate the lexer.
4. is a valid floating-point number it all programming languages I know of, including Python.
@SvenMarnach One current programming language that is an exception is C#. It makes
4 a valid literal of type
4.0 a valid literal of type
4. two separate tokens. This way,
double d = 4.; is disallowed, but the user can simply write
double d = 4.0;. This way,
string s = 4.ToString();
is allowed. (You did say "all programming languages [you] know of", so you're not wrong if you don't know of C#, but it's probably a useful addition for others.)