The "advantage" of from xyz import *
as opposed to other forms of import is that it imports everything (well, almost... [see (a) below] everything) from the designated module under the current module. This allows using the various objects (variables, classes, methods...) from the imported module without prefixing them with the module's name. For example
>>> from math import *
>>>pi
3.141592653589793
>>>sin(pi/2)
>>>1.0
This practice (of importing * into the current namespace) is however discouraged because it
- provides the opportunity for namespace collisions (say if you had a variable name pi prior to the import)
- may be inefficient, if the number of objects imported is big
- doesn't explicitly document the origin of the variable/method/class (it is nice to have this "self documentation" of the program for future visit into the code)
Typically we therefore limit this import * practice to ad-hoc tests and the like. As pointed out by @Denilson-Sá-Maia, some libraries such as (e.g. pygame) have a sub-module where all the most commonly used constants and functions are defined and such sub-modules are effectively designed to be imported with import *
. Other than with these special sub-modules, it is otherwise preferable to ...:
explicitly import a few objects only
>>>from math import pi
>>>pi
>>>3.141592653589793
>>> sin(pi/2)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'sin' is not defined
or import the module under its own namespace (or an alias thereof, in particular if this is a long name, and the program references its objects many times)
>>>import math
>>>math.pi
>>>3.141592653589793
etc..
>>>import math as m #bad example math being so short and standard...
>>>m.pi
>>>3.141592653589793
etc..
See the Python documentation on this topic
(a) Specifically, what gets imported with from xyz import *
?
if xyz module defines an __all__
variable, it will import all the names defined in this sequence, otherwise it will import all names, except these which start with an underscore.
Note Many libraries have sub-modules. For example the standard library urllib
includes sub-modules like urllib.request
, urllib.errors
, urllib.response
etc. A common point of confusion is that
from urllib import *
would import all these sub-modules. That is NOT the case: one needs to explicitly imports these separately with, say, from urllib.request import *
etc. This incidentally is not specific to import *
, plain import
will not import sub-modules either (but of course, the *
which is often a shorthand for "everything" may mislead people in thinking that all sub-modules and everything else would be imported).
Best Answer
I believe by "in the middle of your program" you are talking about an import inside a function definition:
This is not allowed because it would make optimizing the body of the function too hard. The Python implementation wants to know all of the names of function-local variables when it byte-compiles a function, so that it can optimize variable references into operations on the (CPython) virtual machine's operand stack, or at least to local variable-slot operations rather than lookups in outer namespaces. If you could dump the entire contents of a module into a function's local namespace, then the compiler would have to assume that any name in the function might possibly refer to a module global, because the list of names brought in by
from module import *
is only known at runtime.Putting
from module import *
in between top-level declarations is poor style, but it's allowed:EDIT April 2013: While looking into something else, I discovered that this restriction was introduced in Python 2.1, as a consequence of the "Nested Scopes" feature (PEP 227). Quoting from the link:
This clarifies the Python 3.x vs 2.x behavior discussed in the comments. It is always contrary to the language specification, but CPython 2.1 through 2.7 only issue an error for
from module import *
within a function if it might affect the compiler's ability to know whether a variable binds locally or in a containing scope. In 3.x it has been promoted to an unconditional error.SON OF EDIT: ... and apparently flashk pointed this out years ago in another answer, quoting the same paragraph of "What's New in Python 2.1" yet. Y'all go upvote that now.