In Python, what is the difference between expressions and statements?
This question is related to
python
expression
A statement contains a keyword.
An expression does not contain a keyword.
print "hello"
is statement, because print
is a keyword.
"hello"
is an expression, but list compression is against this.
The following is an expression statement, and it is true without list comprehension:
(x*2 for x in range(10))
An expression is something that can be reduced to a value, for example "1+3"
is an expression, but "foo = 1+3"
is not.
It's easy to check:
print(foo = 1+3)
If it doesn't work, it's a statement, if it does, it's an expression.
Another statement could be:
class Foo(Bar): pass
as it cannot be reduced to a value.
An expression is something, while a statement does something.
An expression is a statement as well, but it must have a return.
>>> 2 * 2 #expression
>>> print(2 * 2) #statement
PS:The interpreter always prints out the values of all expressions.
STATEMENT:
A Statement is a action or a command that does something. Ex: If-Else,Loops..etc
val a: Int = 5
If(a>5) print("Hey!") else print("Hi!")
EXPRESSION:
A Expression is a combination of values, operators and literals which yields something.
val a: Int = 5 + 5 #yields 10
An expression translates to a value.
A statement consumes a value* to produce a result**.
*That includes an empty value, like: print()
or pop()
.
**This result can be any action that changes something; e.g. changes the memory ( x = 1) or changes something on the screen ( print("x") ).
A few notes:
Expressions always evaluate to a value, statements don't.
e.g.
variable declaration and assignment are statements because they do not return a value
const list = [1,2,3];
Here we have two operands - a variable 'sum' on the left and an expression on the right. The whole thing is a statement, but the bit on the right is an expression as that piece of code returns a value.
const sum = list.reduce((a, b)=> a+ b, 0);
Function calls, arithmetic and boolean operations are good examples of expressions.
Expressions are often part of a statement.
The distinction between the two is often required to indicate whether we require a pice of code to return a value.
Expressions:
objects
and operators
.<object><operator><object>
2.0 + 3
is an expression which evaluates to 5.0
and has a type float
associated with it.
Statements
Statements are composed of expression(s). It can span multiple lines.
Statements represent an action or command e.g print statements, assignment statements.
print 'hello', x = 1
Expression is a combination of variables, operations and values that yields a result value.
5 * 5 # yields 25
Lastly, expression statements
print 5*5
Though this isn't related to Python:
An expression
evaluates to a value.
A statement
does something.
>>> x + 2 # an expression
>>> x = 1 # a statement
>>> y = x + 1 # a statement
>>> print y # a statement (in 2.x)
2
Python calls expressions "expression statements", so the question is perhaps not fully formed.
A statement consists of pretty much anything you can do in Python: calculating a value, assigning a value, deleting a variable, printing a value, returning from a function, raising an exception, etc. The full list is here: http://docs.python.org/reference/simple_stmts.html#
An expression statement is limited to calling functions (e.g., math.cos(theta)"), operators ( e.g., "2+3"), etc. to produce a value.
Statements before could change the state of our Python program: create or update variables, define function, etc.
And expressions just return some value can't change the global state or local state in a function.
But now we got :=
, it's an alien!
Expression -- from the New Oxford American Dictionary:
expression: Mathematics a collection of symbols that jointly express a quantity : the expression for the circumference of a circle is 2pr.
In gross general terms: Expressions produce at least one value.
In Python, expressions are covered extensively in the Python Language Reference In general, expressions in Python are composed of a syntactically legal combination of Atoms, Primaries and Operators.
Python expressions from Wikipedia
Examples of expressions:
Literals and syntactically correct combinations with Operators and built-in functions or the call of a user-written functions:
>>> 23
23
>>> 23l
23L
>>> range(4)
[0, 1, 2, 3]
>>> 2L*bin(2)
'0b100b10'
>>> def func(a): # Statement, just part of the example...
... return a*a # Statement...
...
>>> func(3)*4
36
>>> func(5) is func(a=5)
True
Statement from Wikipedia:
In computer programming a statement can be thought of as the smallest standalone element of an imperative programming language. A program is formed by a sequence of one or more statements. A statement will have internal components (e.g., expressions).
Python statements from Wikipedia
In gross general terms: Statements Do Something and are often composed of expressions (or other statements)
The Python Language Reference covers Simple Statements and Compound Statements extensively.
The distinction of "Statements do something" and "expressions produce a value" distinction can become blurry however:
if
is usually a statement, such as if x<0: x=0
but you can also have a conditional expression like x=0 if x<0 else 1
that are expressions. In other languages, like C, this form is called an operator like this x=x<0?0:1;
def func(a): return a*a
is an expression when used but made up of statements when defined. None
is a procedure in Python: def proc(): pass
Syntactically, you can use proc()
as an expression, but that is probably a bug...func(x=2);
Is that an Expression or Statement? (Answer: Expression used as a Statement with a side-effect.) The assignment statement of x=2
inside of the function call of func(x=2)
in Python sets the named argument a
to 2 only in the call to func
and is more limited than the C example. An expression is a combination of values, variables, and operators. A value all by itself is considered an expression, and so is a variable, so the following are all legal expressions:
>>> 42
42
>>> n
17
>>> n + 25
42
When you type an expression at the prompt, the interpreter evaluates it, which means that it finds the value of the expression. In this example, n has the value 17 and n + 25 has the value 42.
A statement is a unit of code that has an effect, like creating a variable or displaying a value.
>>> n = 17
>>> print(n)
The first line is an assignment statement that gives a value to n. The second line is a print statement that displays the value of n. When you type a statement, the interpreter executes it, which means that it does whatever the statement says. In general, statements don’t have values.
Source: Stackoverflow.com