difference between statement and expression

[akbarza]

hi
On the site realpython.com, I saw that was said that yield is a statement.
what is the difference between a statement and an expression in Python? plz, explain.
thanks

[perfringo]

It's always a good idea to start with documentation:

Expression:

A piece of syntax which can be evaluated to some value. In other words, an expression is an accumulation of expression elements like literals, names, attribute access, operators or function calls which all return a value. In contrast to many other languages, not all language constructs are expressions. There are also statements which cannot be used as expressions, such as while. Assignments are also statements, not expressions.

Statement:

A statement is part of a suite (a “block” of code). A statement is either an expression or one of several constructs with a keyword, such as if, while or for.

[akbarza]

It's always a good idea to start with documentation:

Expression:

A piece of syntax which can be evaluated to some value. In other words, an expression is an accumulation of expression elements like literals, names, attribute access, operators or function calls which all return a value. In contrast to many other languages, not all language constructs are expressions. There are also statements which cannot be used as expressions, such as while. Assignments are also statements, not expressions.

Statement:

A statement is part of a suite (a “block” of code). A statement is either an expression or one of several constructs with a keyword, such as if, while or for.

hi
is it true that expression is oneline code, while statement is a block code?
i read above , but i can not understand the difference.

[deanhystad]

All code are statements. Only code that returns a value is an expression.

i = 0  # Statement
i == 0  # Statement and expression
while i < 5:   # A statment that contains an expression, i < 5 is an expression
    print(i)  # only a statement.  No returned value
    i = i + 1  # A statement,  i + 1 is an expression

[buran]

All code are statements.

print(i)  # only a statement.  No returned value

This is not correct. As mentioned in the definition, referenced by @perfringo

an expression is an accumulation of expression elements like literals, names, attribute access, operators or function calls which all return a value.

In python3 print is a function. When call print(i) it returns (i.e. evaluates to) None.
Any function call, incl. call of print() is an expression. In your example (and normally) we just throw away the value None as we don't care.
Compare with python2 where print is indeed statement

python3

>>> spam = print('eggs')
eggs
>>> spam is None
True
>>>

python2

>>> spam = print 'eggs'
  File "<stdin>", line 1
    spam = print 'eggs'
                 ^
SyntaxError: invalid syntax

[Gribouillis]

Why not use the Python grammar specification to understand what a statement is and what an expression is? An expression is a special kind of 'simple_stmt' called a 'star_expressions'.

Also read the Python language reference.

[akbarza]

thanks all