Use the import statement:

>>> import random
>>> print(random.randint(1, 10))
4

import module will import a module and then allow you to reference its objects -- values, functions and classes, for example -- using the module.name syntax. In the above example, the random module is imported, which contains the randint function. So by importing random you can call randint with random.randint.

You can import a module and assign it to a different name:

>>> import random as rn
>>> print(rn.randint(1, 10))
4

If your python file main.py is in the same folder as custom.py. You can import it like this:

import custom

It is also possible to import a function from a module:

>>> from math import sin
>>> sin(1)
0.8414709848078965

To import specific functions deeper down into a module, the dot operator may be used only on the left side of the import keyword:

from urllib.request import urlopen

In python, we have two ways to call function from top level. One is import and another is from. We should use import when we have a possibility of name collision. Suppose we have hello.py file and world.py files having same function named function. Then import statement will work good.

from hello import function
from world import function

function() #world's function will be invoked. Not hello's  

In general import will provide you a namespace.

import hello
import world

hello.function() # exclusively hello's function will be invoked 
world.function() # exclusively world's function will be invoked

But if you are sure enough, in your whole project there is no way having same function name you should use from statement

Multiple imports can be made on the same line:

>>> # Multiple modules
>>> import time, sockets, random
>>> # Multiple functions
>>> from math import sin, cos, tan
>>> # Multiple constants
>>> from math import pi, e

>>> print(pi)
3.141592653589793
>>> print(cos(45))
0.5253219888177297
>>> print(time.time())
1482807222.7240417

The keywords and syntax shown above can also be used in combinations:

>>> from urllib.request import urlopen as geturl, pathname2url as path2url, getproxies
>>> from math import factorial as fact, gamma, atan as arctan
>>> import random.randint, time, sys

>>> print(time.time())
1482807222.7240417
>>> print(arctan(60))
1.554131203080956
>>> filepath = "/dogs/jumping poodle (december).png"
>>> print(path2url(filepath))
/dogs/jumping%20poodle%20%28december%29.png

Instead of importing the complete module you can import only specified names:

from random import randint # Syntax "from MODULENAME import NAME1[, NAME2[, ...]]"
print(randint(1, 10))      # Out: 5

from random is needed, because the python interpreter has to know from which resource it should import a function or class and import randint specifies the function or class itself.

Another example below (similar to the one above):

from math import pi
print(pi)                  # Out: 3.14159265359

The following example will raise an error, because we haven't imported a module:

random.randrange(1, 10)    # works only if "import random" has been run before

Outputs:

NameError: name 'random' is not defined

The python interpreter does not understand what you mean with random. It needs to be declared by adding import random to the example:

import random
random.randrange(1, 10)

from module_name import *

for example:

from math import *
sqrt(2)    # instead of math.sqrt(2)
ceil(2.7)  # instead of math.ceil(2.7)

This will import all names defined in the math module into the global namespace, other than names that begin with an underscore (which indicates that the writer feels that it is for internal use only).

Warning: If a function with the same name was already defined or imported, it will be overwritten. Almost always importing only specific names from math import sqrt, ceil is the recommended way:

def sqrt(num):
    print("I don't know what's the square root of {}.".format(num))

sqrt(4)
# Output: I don't know what's the square root of 4.

from math import * 
sqrt(4)
# Output: 2.0

Starred imports are only allowed at the module level. Attempts to perform them in class or function definitions result in a SyntaxError.

def f():
    from math import *

and

class A:
    from math import *

both fail with:

SyntaxError: import * only allowed at module level

Modules can have a special variable named __all__ to restrict what variables are imported when using from mymodule import *.

Given the following module:

# mymodule.py

__all__ = ['imported_by_star']

imported_by_star = 42
not_imported_by_star = 21

Only imported_by_star is imported when using from mymodule import *:

>>> from mymodule import *
>>> imported_by_star
42
>>> not_imported_by_star
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'not_imported_by_star' is not defined

However, not_imported_by_star can be imported explicitly:

>>> from mymodule import not_imported_by_star
>>> not_imported_by_star
21

import importlib
random = importlib.import_module("random")

collections_abc = importlib.import_module("collections.abc")

For older versions of Python, use the imp module.

import imp, sys
def import_module(name):
    fp, pathname, description = imp.find_module(name)
    try:
        return imp.load_module(name, fp, pathname, description)
    finally:
        if fp:
            fp.close()

Do NOT use __import__() to programmatically import modules! There are subtle details involving sys.modules, the fromlist argument, etc. that are easy to overlook which importlib.import_module() handles for you.

If you want to import a module that doesn't already exist as a built-in module in the Python Standard Library nor as a side-package, you can do this by adding the path to the directory where your module is found to sys.path. This may be useful where multiple python environments exist on a host.

import sys
sys.path.append("/path/to/directory/containing/your/module")
import mymodule

It is important that you append the path to the directory in which mymodule is found, not the path to the module itself.

Some recommended PEP8 style guidelines for imports:

1. Imports should be on separate lines:

    from math import sqrt, ceil      # Not recommended
    from math import sqrt            # Recommended
    from math import ceil
   

2. Order imports as follows at the top of the module:

   • Standard library imports
   • Related third party imports
   • Local application/library specific imports

3. Wildcard imports should be avoided as it leads to confusion in names in the current namespace. If you do from module import *, it can be unclear if a specific name in your code comes from module or not. This is doubly true if you have multiple from module import *-type statements.

4. Avoid using relative imports; use explicit imports instead.

from module.submodule import function

This imports function from module.submodule.

The __import__() function can be used to import modules where the name is only known at runtime

if user_input == "os":
    os = __import__("os")

# equivalent to import os

This function can also be used to specify the file path to a module

mod = __import__(r"C:/path/to/file/anywhere/on/computer/module.py")

When using the interactive interpreter, you might want to reload a module. This can be useful if you're editing a module and want to import the newest version, or if you've monkey-patched an element of an existing module and want to revert your changes.

Note that you can't just import the module again to revert:

import math
math.pi = 3
print(math.pi)    # 3
import math
print(math.pi)    # 3

This is because the interpreter registers every module you import. And when you try to reimport a module, the interpreter sees it in the register and does nothing. So the hard way to reimport is to use import after removing the corresponding item from the register:

print(math.pi)    # 3
import sys
if 'math' in sys.modules:  # Is the ``math`` module in the register?
    del sys.modules['math']  # If so, remove it.
import math
print(math.pi)    # 3.141592653589793

But there is more a straightforward and simple way.

Python 2

Use the reload function:

import math
math.pi = 3
print(math.pi)    # 3
reload(math)
print(math.pi)    # 3.141592653589793

Python 3

The reload function has moved to importlib:

import math
math.pi = 3
print(math.pi)    # 3
from importlib import reload
reload(math)
print(math.pi)    # 3.141592653589793