Statements and control flow[edit]
- The assignment statement (token '=', the equals sign). This operates differently than in traditional imperative programming languages, and this fundamental mechanism (including the nature of Python's version of "variables") illuminates many other features of the language. Assignment in C, e.g., "x = 2", translates to "typed variable name x receives a copy of numeric value 2". The (right-hand) value is copied into an allocated storage location for which the (left-hand) variable name is the symbolic address. The memory allocated to the variable is large enough (potentially quite large) for the declared type. In the simplest case of Python assignment, using the same example, "x = 2", translates to "(generic) name x receives a reference to a separate, dynamically allocated object of numeric (int) type of value 2." This is referred to as "binding" the name to the object. Since the name's storage location doesn't "contain" the indicated value, it is not proper to refer to it as a "variable." Names may be subsequently re-bound at any time to objects of greatly varying types, including strings, procedures, complex objects with data and methods, etc. Successive assignments of a common value to multiple names, e.g., "x = 2"; "y = 2"; "z = 2" result in allocating storage to (at most) three names and a single numeric object, to which all three names are bound. Since a name is a generic reference holder it is not reasonable to associate a fixed data type with it. However at a given time a name will be bound to some object, which will have a type; thus there is dynamic typing.
- The
if statement, which conditionally executes a block of code, along with else and elif (a contraction of else-if).
- The
for statement, which iterates over an iterable object, capturing each element to a local variable for use by the attached block.
- The
while statement, which executes a block of code as long as its condition is true.
- The
try statement, which allows exceptions raised in its attached code block to be caught and handled by except clauses; it also ensures that clean-up code in afinally block will always be run regardless of how the block exits.
- The
class statement, which executes a block of code and attaches its local namespace to a class, for use in object-oriented programming.
- The
def statement, which defines a function or method.
- The
with statement (from Python 2.5), which encloses a code block within a context manager (for example, acquiring a lock before the block of code is run and releasing the lock afterwards, or opening a file and then closing it), allowing RAII-like behavior.
- The
pass statement, which serves as a NOP. It is syntactically needed to create an empty code block.
- The
assert statement, used during debugging to check for conditions that ought to apply.
- The
yield statement, which returns a value from a generator function. From Python 2.5, yield is also an operator. This form is used to implement coroutines.
- The
import statement, which is used to import modules whose functions or variables can be used in the current program.
- The
print statement was changed to the print() function in Python 3.[54]
if statement, which conditionally executes a block of code, along with else and elif (a contraction of else-if).for statement, which iterates over an iterable object, capturing each element to a local variable for use by the attached block.while statement, which executes a block of code as long as its condition is true.try statement, which allows exceptions raised in its attached code block to be caught and handled by except clauses; it also ensures that clean-up code in afinally block will always be run regardless of how the block exits.class statement, which executes a block of code and attaches its local namespace to a class, for use in object-oriented programming.def statement, which defines a function or method.with statement (from Python 2.5), which encloses a code block within a context manager (for example, acquiring a lock before the block of code is run and releasing the lock afterwards, or opening a file and then closing it), allowing RAII-like behavior.pass statement, which serves as a NOP. It is syntactically needed to create an empty code block.assert statement, used during debugging to check for conditions that ought to apply.yield statement, which returns a value from a generator function. From Python 2.5, yield is also an operator. This form is used to implement coroutines.import statement, which is used to import modules whose functions or variables can be used in the current program.print statement was changed to the print() function in Python 3.[54]
Python's statements include (among others):
Python does not support tail-call optimization or first-class continuations, and, according to Guido van Rossum, it never will.[55][56] However, better support for coroutine-like functionality is provided in 2.5, by extending Python's generators.[57] Prior to 2.5, generators were lazy iterators; information was passed unidirectionally out of the generator. As of Python 2.5, it is possible to pass information back into a generator function, and as of Python 3.3, the information can be passed through multiple stack levels.[58]
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