The Question :
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In Python, what is the difference between expressions and statements?
The Question Comments :
The Answer 1
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Expressions only contain identifiers, literals and operators, where operators include arithmetic and boolean operators, the function call operator
() the subscription operator
 and similar, and can be reduced to some kind of “value”, which can be any Python object. Examples:
3 + 5
map(lambda x: x*x, range(10))
[a.x for a in some_iterable]
Statements (see 1, 2), on the other hand, are everything that can make up a line (or several lines) of Python code. Note that expressions are statements as well. Examples:
# all the above expressions
if x: do_y()
a = 7
The Answer 2
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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 2πr.
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:
[0, 1, 2, 3]
>>> def func(a): # Statement, just part of the example...
... return a*a # Statement...
>>> func(5) is func(a=5)
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.,
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:
- List Comprehensions are considered “Expressions” but they have looping constructs and therfore also Do Something.
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
- You can write you own Expressions by writing a function.
def func(a): return a*a is an expression when used but made up of statements when defined.
- An expression that returns
None is a procedure in Python:
def proc(): pass Syntactically, you can use
proc() as an expression, but that is probably a bug…
- Python is a bit more strict than say C is on the differences between an Expression and Statement. In C, any expression is a legal statement. You can have
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.
The Answer 3
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Though this isn’t related to Python:
expression evaluates to a value.
statement does something.
>>> x + 2 # an expression
>>> x = 1 # a statement
>>> y = x + 1 # a statement
>>> print y # a statement (in 2.x)
The Answer 4
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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
The Answer 5
10 people think this answer is useful
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.
The Answer 6
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- An expression is a statement that returns a value. So if it can appear on the right side of an assignment, or as a parameter to a method call, it is an expression.
- Some code can be both an expression or a statement, depending on the context. The language may have a means to differentiate between the two when they are ambiguous.
The Answer 7
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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.
The Answer 8
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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!")
A Expression is a combination of values, operators and literals which yields something.
val a: Int = 5 + 5 #yields 10
The Answer 9
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Expressions always evaluate to a value, statements don’t.
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.
The Answer 10
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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!
The Answer 11
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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))
The Answer 12
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- Expressions are formed by combining
- An expression has a value, which has a type.
- Syntax for a simple expression:
2.0 + 3 is an expression which evaluates to
5.0 and has a type
float associated with it.
Statements are composed of expression(s). It can span multiple lines.
The Answer 13
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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:
>>> n + 25
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
A statement is a unit of code that has an effect, like creating a variable or displaying a
>>> n = 17
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.
This can be useful – thinkpython2 by Allen B. Downey
The Answer 14
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An expression translates to a value.
A statement consumes a value* to produce a result**.
*That includes an empty value, like:
**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:
- Since a statement can return a result, it can be part of an expression.
- An expression can be part of another expression.
The Answer 15
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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.