Importing functions and packages

Importing functions and packages#

Very often, we need tools that are not directly provided by the default Python library. In that case we need to import the necessary functions from modules or packages into our notebooks or scripts. Some modules come directly with the Python distribution (like the math module seen in the Basic math in python), and some others like Numpy, scikit-image etc. are external packages installed with pip or conda. When it comes to import they work in the same way, and we have multiple ways of importing single functions or groups of them.

Basic import#

The basic import statment uses the key-words import and the module name. For example with the basic Python module pathlib that deals with file paths and names:

import pathlib

If we want to import an external package we need to make sure it is actually installed, otherwise we get an error message:

import absent_package

---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
Input In [14], in <cell line: 1>()
----> 1 import absent_package

ModuleNotFoundError: No module named 'absent_package'

If you have a missing package, you can install it directly from the notebook using pip or conda. For example if Numpy is not installed yet you could execute:

conda install -c conda-forge numpy

Collecting package metadata (current_repodata.json): done
Solving environment: done

# All requested packages already installed.


Note: you may need to restart the kernel to use updated packages.

Alternative import formulation#

Using the simple formulation above, we get access to functions directly attached to the main package. Let’s import the Numpy package that we will use later to handle image. We can import it:

import numpy

and now we get e.g. access to the cosine function of Numpy by simply using the dot notation:

numpy.cos(3.14)

-0.9999987317275395

To find all functions available in a package or a module, you typically have to go to its documentation and look for the Application Programming Interface or API. For example here we find a description of all mathematical functions (including the cosine) of Numpy: https://numpy.org/doc/stable/reference/routines.math.html

Name shortening#

If we use a package regularly, we might not want to write the full package name every time we need a function from it. To avoid that we can abbreviate it at the time of import using the as statement:

import numpy as np

np.cos(3.14)

-0.9999987317275395

Specific functions#

If we only need a specific function from a package, we can also just import it using the from statement. For examample if we want to only import the np.cos function above:

from numpy import cos

cos(3.14)

-0.9999987317275395

Of course in this case, unless we find the specfiic line where the function is imported, we don’t know that cos necessarily belongs to Numpy as we could also have defined such a function in the notebook.

Sub-modules#

In larger packages like Numpy, some functions are directly accessible from the main package (like np.cos) and others with more specialized tasks are grouped together by topic or domain into submodules. For example Numpy has a submodule dedicated to distributions called random. All the points seen above are still valide here.

We use the dot notation to access functions, but now need to also specify the submodule name. For example the normal function that generates numbers drawn from a normal distribution:

np.random.normal()

1.8077301364791438

We can shorten the function call by importing only the sub-module:

from numpy import random

random.normal()

0.0909924357071552

and we can further shorten by importing just the function:

from numpy.random import normal

normal()

0.0038794083334154307

Exercise#

The Numpy package has a linear algebra submodule called linalg. The following code computes the norm of a vector: np.linalg.norm([1,2]). Try to:

  • import just the sub-module and call the same function

  • import just the norm function and call it

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