How To Check For Unwanted Characters Using Loops With Python

Estimated reading time: 3 minutes

On this website, we have posted about how to remove unwanted characters from your data and How to remove characters from an imported CSV file both will show you different approaches.

In this blog posting, we are going to approach the process by using loops within a function. Essentially we are going to pass a list and then we are going to loop through the strings to check the data against it.

Step 1 – Import the data and create a data frame

The first bit of work we need to complete is to load the data. Here we create a dictionary with their respective key-value pairs.

In order to prepare the data to be processed through the function in step 2, we then load it into a data frame.

import pandas as pd
#Create a dataset locally
data = {'Number':  ["00&00$000", '111$11111','2222€2222','3333333*33','444444444£','555/55555','666666@666666'],
        'Error' : ['0','0','0','0','0','0','0']}

#Create a dataframe and tell it where to get its values from
df = pd.DataFrame (data, columns = ['Number','Error'])

Step 2 – Create the function that checks for invalid data

This is the main piece of logic that gives the output. As you can see there is a list “L” that is fed to the function run.

One thing to note is that *l is passed to the function, as there is more than one value in the list, otherwise the program would not execute properly.

To start off we create a data frame, which extracts using a regular expression the characters we don’t want to have.

Next, we then need to drop a column that is generated with NAN values, as these are not required.

Then we updated the original data fame with the values that we found.

Just in case if there are any NAN values in this updated column “Error”, we remove them on the next line.

The main next is the loop that creates a new column called “Fix”. This holds the values that will be populated into the fix after the data we don’t want is removed and is data cleansed.

The data we do not need is in str.replace.

#Function to loop through the dataset and see if any of the list values below are found and replace them
def run(*l):
    #This line extracts all the special characters into a new column
    #Using regular expressions it finds values that should not appear
    df2 = df['Number'].str.extract('(\D+|^(0)$)') # New dataframe to get extraction we need
    df2 = df2.drop(1, axis=1) # Drops the column with NAN in it, not required

    df['Error'] = df2[0] # Updates original dataframe with values that need to be removed.
    #This line removes anything with a null value
    df.dropna(subset=['Error'], inplace=True)
    #This line reads in the list and assigns it a value i, to each element of the list.
    #Each i value assigned also assigns an index number to the list value.
    #The index value is then used to check whether the value associated with it is in the df['Number'] column 
    #and then replaces if found
    for i in l:
        df['Fix']= df['Number'].str.replace(i[0],"").str.replace(i[1],"").str.replace(i[2],"").str.replace(i[3],"") \
        print("Error list to check against")

#This is the list of errors you want to check for
l = ['$','*','€','&','£','/','@']

Step 3 – Run the program

To run this program, we just execute the below code. All this does is read in the list “L” to the function “run” and then the output in step 4 is produced


Step 4 – Output

Error list to check against
          Number Error           Fix
0      00&00$000     &       0000000
1      111$11111     $      11111111
2      2222€2222     €      22222222
3     3333333*33     *     333333333
4     444444444£     £     444444444
5      555/55555     /      55555555
6  666666@666666     @  666666666666

Process finished with exit code 0

How to data cleanse a database table

In Data Analytics, that is a very relevant question, and something I look to implement in most projects, sometimes it is too easy to click the shortcut icon to your spreadsheet application!

Here we are looking to bring a little automation into these videos. Building on How to import data from files and Removing characters from an imported CSV file this video connects to a Microsoft Azure cloud database table, brings in the data with errors on it, fixes the errors and displays the correct output on the screen.

What can this do for organisations?

There are several benefits to automating this step:

  • Less manual intervention if there is a need to fix data issues.
  • Better productivity.
  • Better data flows with no errors and quicker reporting.


Moving away from files

The process of moving away from files and into automation has several steps:

  • Be clear on your data needs.
  • Understand what you are trying to achieve.
  • Build a process that is repeatable but can be updated easily.
  • Ensure that you build in data quality checks, helps deliver the better output to the users.

Thanks for stopping by!

Data Analytics Ireland


How to remove unwanted characters

Estimated reading time: 2 minutes

Removing the unwanted, that is holding you up.
A situation has arisen of you having information, which has erroneous data inside it, what do you do?

Data issues are a common scenario faced by many data analytics professionals and the industry as a whole. Data quality now has become more critical, especially as we move more processes online and the digital landscape increases.

Most pieces of data go through a process of been transferred somewhere between systems to be used or reports rely on the accuracy of them. If the data in the source system has quality issues, the problem if not addressed before going somewhere else can then push the data quality issues more throughout an organisation, like a spiders web it expands further.

The next step, looking to fix the problem and planning for it.

To combat this problem professionals need to come up with a plan on how to tackle this, either:

  • Fix at source
  • Take the data in before moving it on, and investigate the problems.
  •  Reject the file or part thereof.

All three options above have scenarios around them with costs and implications, depending on the industry, you need to pick the most appropriate way to handle. As an example, in the banking industry payment files can sometimes have data in them that is rejected entirely or in part.

But the bank may decide they will only discard the records with the wrong data and process everything else.

How to go about it and how regular expressions can help

In this video, we look to go through an example of how to cleanse a data set;

(A) We use a list to check what problems we need to find.

(B) Using functions again to process through the data to find the problem and extract them.

(C) Regular expressions also appear as they look to find the special characters in the data set.

The concept of regular expressions is used extensively across several programming languages; it is a good way to test data and find erroneous values. If you are thinking about machine learning, it is quite important to get a more thorough knowledge of how they work. Here is a good link for further reading if you need more information Regular Expression how to

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Data Analytics Ireland

YouTube channel lists – Python Data Cleansing

Ever had a process where you received a set of data, and it took a bit of effort to cleanse the data, so it looks the way you want it?

The world of data processing and data exchanged between servers and organisations needs careful attention, one person’s idea of clean data might not be other persons, hence the difference between the two can lead to data issues.

Experian had an excellent article What is data cleansing? in that they talk about several factors about data:

  • It could be incorrect
  • and incomplete
  •  and Duplicated


One of the things they highlighted also is that under GDPR, organisations have to deal with more focus on data been accurate, complete and up to date.

We are putting together several videos in this area over time, so you will start to see them as they go up.

Please like and share through the social media buttons shared on the page here, thanks for watching!

Data Analytics Ireland