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11 Modules

question Questions
  • How to import functions from Python's built-in libraries and from other people or from yourself
objectives Objectives
  • Understand how to import modules in several ways and how to call functions within a module.
  • Use built-in modules from Python, e.g. to access date and time, or to navigate in your folders,...

time Time estimation: 40 minutes

11.1 Introduction

So now that we know how to make functions, how can you re-use them? Imagine that you’ve started writing code and functions in one file and the project has grown to such an extent that it would be easier to maintain it in different files each containing a specific part of the project. Or you want to re-use some of the functions in other projects as well.

In Python you can import functions and chunks of code from files. Such a file containing the functions is called a module. Generally we say that we import a definition from a module. A module can have one or multiple functions in it. The file name is the module name with the suffix .py appended.

Using the code from this module is possible by using import. In this way you can import your own functions, but also draw on a very extensive library of functions provided by Python (built-in modules). We will first look at the syntax for imports and how to import your own functions, then explore the most commonly used Python libraries.

11.2 How imports work

The easiest way to import a module looks like this:

import module1

Imagine that in the module module1, there is a function called getMeanValue(). This way of importing does not make the name of the function available; it only remembers the module name module1 which you can than use to access the functions within the module:

import module1

11.3 How to create your own module

The easiest example is importing a module from within the same working directory. Let’s create a Python module called with the code of the function getMeanValue() that we have written earlier (and you can find here below).

hands_on Create your own module

To create your own module from Jupyter Notebook, follow these steps:

  1. In order to create a module in Jupyter Lab, first create a new notebook
  2. Rename the notebook (e.g. ‘module1.ipynb’) and copy paste the code in the notebook
  3. Click ‘File’, ‘Download as’ and ‘Python’
  4. Jupyter will not download it in some local folder, copy it to your current working directory (in our case in the same directory as we’re in right now).

Unfortunately, Jupyter Notebook doesn’t have a streamlined & straightforward way of creating Python modules and Python scripts. When you export the notebook, it will always export the whole Notebook and not just a part of it, which makes it very messy if you have a very large notebook.

Import the following code in the file.

# When you download this as a Python script, Jupyter will automatically insert the environment shebang here. 

def getMeanValue(valueList):
    Calculate the mean (average) value from a list of values.
    Input: list of integers/floats
    Output: mean value
    valueTotal = 0.0
    for value in valueList:
        valueTotal += value
    numberValues = len(valueList)
    return (valueTotal/numberValues)

11.4 Import syntax

We can now use the module we just created by importing it. In this case where we import the whole ‘module1’ file, we can call the function as a method, similar to the methods for lists and strings that we saw earlier:

import module1


If we were to write code for a huge project, long names can get exhaustive. Programmers will intrinsically make shortcut names for functions they use a lot. Renaming a module is therefore a common thing to do (e.g. NumPy as np, pandas as pd, etc.):

import module1 as m1


When importing a file, Python only searches the current directory, the directory that the entry-point script is running from, and sys.path which includes locations such as the package installation directory (it’s actually a little more complex than this, but this covers most cases).

However, you can specify the Python path yourself as well. If you’re using the materials from Github, note that within our folders there is a directory named modules and within this folder, there is a module named module2 (recognizable due to its .py extension). In that module there are two functions: ‘getMeanValue’ and ‘compareMeanValueOfLists’.

from modules import module2

from modules import module2 as m2


Another way of writing this is with an absolute path to the module. You can explicitly import an attribute from a module.

from modules.module2 import compareMeanValueOfLists

print(compareMeanValueOfLists([1,2,3,4,5,6,7], [4,6,77,3,67,54,6,5]))

So here we import the function compareMeanValueOfLists (without brackets!) from the file module2 (without .py extension!).

In order to have an overview of all the different functions within a module, use dir():


11.5 Built-in Modules

There are several built-in modules in Python, which you can import whenever you like.

Python has many ready-to-use functions that can save you a lot of time when writing code. The most common ones are time, sys, os/os.path and re.

11.5.1 time

With time you can get information on the current time and date, …:

import time
time.ctime()  # Print current day and time
time.time()   # Print system clock time
time.sleep(10)       # Sleep for 5 seconds - the program will wait here

See the Python documentation for a full description of time. Also see datetime, which is a module to deal with date/time manipulations.

11.5.2 sys

gives you system-specific parameters and functions:

import sys

sys.argv  # A list of parameters that are given when calling this script 
          # from the command line (e.g. ''python myScript a b c'')
sys.platform # The platform the code is currently running on
sys.path     # The directories where Python will look for things to import
help(sys.exit)          # Exit the code immediately

See the Python documentation for a full description.

11.5.3 os and os.path

are very useful when dealing with files and directories:

import os
# Get the current working directory (cwd)
currentDir = os.getcwd()
# Get a list of the files in the current working directory    
myFiles = os.listdir(currentDir)
# Create a directory, rename it, and remove it
# Create a full path name to the `module2` module in the modules folder
myFileFullPath = os.path.join(currentDir,'modules','')
# Does this file exist?
# How big is the file?
# Split the directory path from the file name
(myDir,myFileName) = os.path.split(myFileFullPath)

See the Python documentation for os and os.path for a full description.

11.5.4 re

A library that is very powerful for dealing with strings is re. It allows you to use regular expressions to examine text - using these is a course in itself, so just consider this simple example:

import re

myText = """Call me Ishmael. Some years ago - never mind how long precisely -
having little or no money in my purse, and nothing particular to interest me on 
shore, I thought I would sail about a little and see the watery part of the 

# Compile a regular expression, 
myPattern = re.compile("(w\w+d)")    # Look for the first word that starts with a w,
                                     # is followed by 1 or more characters (\w+)
                                     # and ends in a d

mySearch =

# mySearch will be None if nothing was found
if mySearch:

See the full Python documentation on regular expressions for more information.

11.6 Putting everything together

hands_on Exercise 11.6.1

Make a new directory in which you write out 5 files with a 2 second delay. Each file should contain the date and time when it was originally written out.

solution Solution
# 1
import time, os

# Create a variable for the directory name
myDir = "timeTest"

# Check whether the directory exists, if not create it
if not os.path.exists(myDir):

# Loop from 1 to 5
for i in range(1,6):

    # Get the current time
    currentTime = time.ctime()

    # Write out the file - use i to give a different name to each
    filePath = os.path.join(myDir,"myFile{}.txt".format(i))

    outFileHandle = open(filePath,'w')    

    print("Written file {}...".format(filePath))

    # Sleep for 2 seconds

hands_on Exercise 11.6.2

Write a function to read in a FASTA file with an RNA sequence and return the RNA sequence (in 3 base unit chunks).

solution Solution
# 2 
import os
def readRnaFastaFile(fileName):
    if not os.path.exists(fileName):
        print("Error: File {} not available!".format(fileName))
        return (None,None,None)

    fconnect = open(fileName)
    lines = fconnect.readlines()

    sequenceInfo = []
    moleculeName = None
    description = None

    # Get information from the first line - ignore the >
    firstLine = lines[0]
    firstLineCols = firstLine[1:].split()
    moleculeName = firstLineCols[0]
    description = firstLine[1:].replace(moleculeName,'').strip()

    # Now get the full sequence out
    fullSequence = ""
    for line in lines[1:]:

        line = line.strip()
        fullSequence += line

    # Divide up the sequence depending on type (amino acid or nucleic acid)
    for seqIndex in range(0,len(fullSequence),3):

    return (moleculeName,description,sequenceInfo)


hands_on Exercise 11.6.3

Write a program where you ask the user for a one-letter amino acid sequence, and print out the three-letter amino acid codes. Download the dictionary from section 8.2 and save it as a module named first.

solution Solution
# 3
# Note how you can import a function (or variable) with a different name for your program!

from modules.SequenceDicts import proteinOneToThree as oneToThreeLetterCodes

oneLetterSeq = input('Give one letter sequence:')
if oneLetterSeq:
    for oneLetterCode in oneLetterSeq:
        if oneLetterCode in oneToThreeLetterCodes.keys():
            print("One letter code '{}' is not a valid amino acid code!".format(oneLetterCode))
    print("You didn't give me any information!")

hands_on Exercise 11.6.4

Write a program where you translate the RNA sequence data/rnaSeq.txt into 3 letter amino acid codes. Use the dictionary from section 8.2 (called myDictionary) and save it as a module named first. You can use the module from the modules folder.

solution Solution
from modules.SequenceDicts import standardRnaToProtein, proteinOneToThree

from modules.readFasta import readRnaFastaFile

(molName,description,sequenceInfo) = readRnaFastaFile("data/rnaSeq.txt")
proteinThreeLetterSeq = []

for rnaCodon in sequenceInfo:

    aaOneLetterCode = standardRnaToProtein[rnaCodon]
    aaThreeLetterCode = proteinOneToThree[aaOneLetterCode]


hands_on Exercise 11.6.5

Write a program that:

  • Has a function readSampleInformationFile() to read the information from this sample data file into a dictionary. Also check whether the file exists.
  • Has a function getSampleIdsForValueRange() that can extract sample IDs from this dictionary. Print the sample IDs for pH 6.0-7.0, temperature 280-290 and volume 200-220 using this function.

    solution Solution
    import os
    def readSampleInformationFile(fileName):
        # Read in the sample information file in .csv (comma-delimited) format
        # Doublecheck if file exists
        if not os.path.exists(fileName):
            print("File {} does not exist!".format(fileName))
            return None
        # Open the file and read the information
        fileHandle = open(fileName)
        lines = fileHandle.readlines()
        # Now read the information. The first line has the header information which
        # we are going to use to create the dictionary!
        fileInfoDict = {}
        headerCols = lines[0].strip().split(',')
        # Now read in the information, use the first column as the key for the dictionary
        # Note that you could organise this differently by creating a dictionary with
        # the header names as keys, then a list of the values for each of the columns.
        for line in lines[1:]:
            line = line.strip()  # Remove newline characters
            cols = line.split(',')
            sampleId = int(cols[0])
            fileInfoDict[sampleId] = {}
            # Don't use the first column, is already the key!
            for i in range(1,len(headerCols)):
                valueName = headerCols[i]
                value = cols[i]
                if valueName in ('pH','temperature','volume'):
                    value = float(value)
                fileInfoDict[sampleId][valueName] = value
        # Return the dictionary with the file information
        return fileInfoDict
    def getSampleIdsForValueRange(fileInfoDict,valueName,lowValue,highValue):
        # Return the sample IDs that fit within the given value range for a kind of value
        #sampleIdList = fileInfoDict.keys()
        sampleIdList = sorted(fileInfoDict.keys())
        sampleIdsFound = []
        for sampleId in sampleIdList:
            currentValue = fileInfoDict[sampleId][valueName]
            if lowValue <= currentValue <= highValue:
        return sampleIdsFound
    if __name__ == '__main__':
        fileInfoDict = readSampleInformationFile("../data/SampleInfo.txt")

keypoints Key points

  • We can build our own module containing several functions and understand how to use them in some other code.
  • We learned some of Python's built-in modules and know how to e.g. make a folder from Python.

Useful literature

Further information, including links to documentation and original publications, regarding the tools, analysis techniques and the interpretation of results described in this tutorial can be found here.

congratulations Congratulations on successfully completing this tutorial!