Modules and Packages


A module is a component providing Python definitions of functions, variables or any Python code we like. All corresponding to a given specific thema. All these definitions are contained in a single Python file.

All the programs we have written so far have been contained in a single .py file, and so they are modules as well as programs. The key difference is that programs are designed to be run, whereas modules are designed to be imported and used by programs.


In order to use a module, just use the import statement

Several syntaxes can be used when importing. For example:

import importable
import importable1, importable2, …, importableN
import importable as preferred_name

Python comes with numerous modules, we already use some in our previous scripts:

>>> import collections
>>> RestrictEnzyme = collections.namedtuple("RestrictEnzyme", "name comment sequence cut end")
>>> import copy
>>> ascii = ['a','b','c']
>>> integer = [1,2,3]
>>> l = [ascii, integer]
>>> l2 = copy.deepcopy(l)

We already seen some other import syntaxes:

from importable import object as preferred_name
from importable import object1, object2, …, objectN
from importable import (object1, object2, object3, object4, object5,
object6, …, objectN)
from importable import *
>>> from decimal import *
>>> getcontext().prec = 6
>>> Decimal(1) / Decimal(7)

The first syntax is called “fully qualified name” as to use something in a module you import, you have to named it with the all.

The second syntax from … import can cause name conflicts since they make the imported objects (variables, functions, data types, or modules) directly accessible.

for instance we need to use some mathematical functions so we imports the math module in our

def pi(seq):
   compute the isoelectric point of a peptide
   ph = 7.2
   return ph

in we need to use both module_1 and some math functions:

from math import pi
from module_1 import *

pi <function pi at 0x7fb8d5d089b0>

pi was defined in both module here pi from math was rebound to the pi function from module_1 so we cannot use the from syntax, we have to use the import and full qualified name.

In the last syntax, the * means “import everything that is not private” ,which in practical termsmeans either that every object in the module is imported except for those whose names begin with a leading __all__

The from importable import * syntax imports all the objects from the module (or all the modules from the package), this could be hundreds of names. In the case of from os.path import *, almost 40 names are imported, including dirname, exists, and split, any of which might be names we would prefer to use for our own variables or functions.

Where should import statements go in our code?

It is common practice to put all the import statements at the beginning of .py files, after the shebang line, and after the module’s documentation. It is recommended

  • importing standard library modules first,
  • then third-party library modules,
  • and finally our own modules.

Where my modules, packages must be placed to be importable?

Now we have a collection of function on sequences and we like to import these functions from the other python scripts or modules.

Modules are mainly stored in files that are searched:

  1. in your current working directory
  2. in PYTHONHOME, where Python has been installed
  3. in a path, i.e a colon (’:’) separated list of file paths, stored in the environment variable PYTHONPATH. You can check this path through the sys.path variable

The to be importable several options are available to us.

  • If we want the module to be available to a particular program, we just need to put in the same directory as the program.
  • If we want to be available to all our programs, there are a few approaches that can be taken.
    • First approach is to put the module in the Python distribution’s site-packages subdirectory. This directory is in the Python path, so any module that is here will always be found.
    • A second approach is to create a directory specifically for the custom modules we want to use for all our programs, and to add it to the PYTHONPATH environment variable.

The second approache have the advantage of keeping our own code separate from the official installation.

Since we can access to the PYTHONPATH via the vaiable path in sys module. sys.path is a list of path where to search modules. So we can manipulate it inside our scripts to add dynamically the directory where we put the modules in the searching path.

import os.path
home = os.path.expanduser("~")
sys.path.insert(0 , os.path.join(home, "python-lib"))

now python will search modules first in a directory named ‘python-lib’ in youre HOME directory.

An other good way install your own modules without modify the central site-package is to use virtualenv.

a module or a script ?

Sometimes we write code that we want to use either as a module or as a script. We can do this using the internal value of variable __name__.

Whenever a module is imported Python creates a variable for the module called __name__ and stores the module’s name in this variable. for instance

>>> import math
>>> print(math.__name__)

except for for the file which is directly executed by python (not imported) __name__ have as value ‘__main__’

print("module_1 = ", __name__)

>>> import module_1
>>> print(module_1.__name__)

so a file can be used as module or as script.

here the module code
this code will be executed all the time

if __name__ == '__main__':
   here the script code
   this part of code will be executed only if
   this file is directly executed by python


A package is simply a directory that contains a set of modules and a file called Suppose, for example, that we had a fictitious set of module files for manipulating sequences write/read fasta file, … and more specific definitions for protein and nucleic sequences. We could keep the modules in the same directory. By putting them in their own subdirectory, say, sequence, they can be kept together. And if we put an empty file in the sequence directory along with them, the directory will become a package:

In some situations it is convenient to load in all of a package’s modules using a single statement. To do this we must edit the package’s file to contain a statement which specifies which modules we want loaded. This statement must assign a list of module names to the special variable __all__. For example, here:

__all__ = [“nucleic”, “protein”]

For the sequence directory to be a package it must have an file, and as noted, this can be empty or could have an __all__ list as a convenience for programmers who want to import using from sequence import *. But it can contain any python code we want. this code will be executed when the package will be imported

lets have build a tree of python modules like following:

     |_ define functions read_fasta, write_fasta, genetic_code , __all__ ...
     |_protein molecular_weight, isolectric
     |_nucleic reverse_comp, translate

in my program I can use these modules like this:

from bio.sequence import read_fasta
import bio.sequence.nucleic

sequence = read_fasta('my_fasta_file')
prot  = bio.sequence.nucleic.translate(sequence)

to specify a module inside a package we just specify the path to the module using the ‘.’ as package or module separator.

As to specify a file path the module “path” can be absolute or relative.

absolute path begin with the name of a package/module
relative path use ‘.’ or ‘..’ to goback from one level.

for instance in my module I can use sequence like this:

from ..sequence import genetic_code

def translate(sequence)
   prot_seq = ''
   for i in range(len(seq)):
      codon = seq[i, i+3]
      prot_seq += genetic_code[codon]


If you use relative import in your module you cannot execute directly your module like python anymore. If you try this python raise the following error

ValueError: Attempted relative import in non-package


In Python 3, implicit relative imports within packages are no longer available. Only absolute imports and explicit relative imports are supported. In addition, star imports (e.g. from x import *) are only permitted in module level code.


Due to efficiency considerations, once you have imported a module, if you attempt to import it again, Python does not raise an error but just does a nohup operation.

Standard Library

Python provide a lot of libraries which are organized in package and modules. The full description of modules are available on the web site (Python 2 , Python 3). When you code in python always refer to these documentations. These libraries are called “standard library”.