In order to write useful programs, we almost always need the ability to check conditions and change the behavior of the program accordingly. Conditional statements give us this ability. The simplest form is the if statement:
if x > 0: print 'x is positive'
The boolean expression after if is called the condition. If it is true, then the indented statement gets executed. If not, nothing happens.
if statements have the same structure as function deﬁnitions: a header followed by an indented body. Statements like this are called compound statements.
There is no limit on the number of statements that can appear in the body, but there has to be at least one. Occasionally, it is useful to have a body with no statements (usually as a place keeper for code you haven’t written yet). In that case, you can use the pass statement, which does nothing.
if x < 0: pass # need to handle negative values!