Write and Save Files in Python¶

Estimated time needed: 25 minutes

Objectives¶

After completing this lab you will be able to:

• Write to files using Python libraries

Table of Contents

• Writing Files
• Appending Files
• Additional File modes
• Copy a File

---

Writing Files

We can open a file object using the method write() to save the text file to a list. To write to a file, the mode argument must be set to w. Let’s write a file Example2.txt with the line: “This is line A”

In [1]:

# Write line to file
exmp2 = 'Example2.txt'
with open(exmp2, 'w') as writefile:
    writefile.write("This is line A")

# Write line to file exmp2 = 'Example2.txt' with open(exmp2, 'w') as writefile: writefile.write("This is line A")

We can read the file to see if it worked:

In [2]:

# Read file

with open(exmp2, 'r') as testwritefile:
    print(testwritefile.read())

# Read file with open(exmp2, 'r') as testwritefile: print(testwritefile.read())

This is line A

We can write multiple lines:

In [3]:

# Write lines to file

with open(exmp2, 'w') as writefile:
    writefile.write("This is line A\n")
    writefile.write("This is line B\n")

# Write lines to file with open(exmp2, 'w') as writefile: writefile.write("This is line A\n") writefile.write("This is line B\n")

The method .write() works similar to the method .readline(), except instead of reading a new line it writes a new line. The process is illustrated in the figure. The different colour coding of the grid represents a new line added to the file after each method call.

You can check the file to see if your results are correct

In [4]:

# Check whether write to file

with open(exmp2, 'r') as testwritefile:
    print(testwritefile.read())

# Check whether write to file with open(exmp2, 'r') as testwritefile: print(testwritefile.read())

This is line A
This is line B

We write a list to a .txt file as follows:

In [5]:

# Sample list of text

Lines = ["This is line A\n", "This is line B\n", "This is line C\n"]
Lines

# Sample list of text Lines = ["This is line A\n", "This is line B\n", "This is line C\n"] Lines

Out[5]:

['This is line A\n', 'This is line B\n', 'This is line C\n']

In [6]:

# Write the strings in the list to text file

with open('Example2.txt', 'w') as writefile:
    for line in Lines:
        print(line)
        writefile.write(line)

# Write the strings in the list to text file with open('Example2.txt', 'w') as writefile: for line in Lines: print(line) writefile.write(line)

This is line A

This is line B

This is line C

We can verify the file is written by reading it and printing out the values:

In [7]:

# Verify if writing to file is successfully executed

with open('Example2.txt', 'r') as testwritefile:
    print(testwritefile.read())

# Verify if writing to file is successfully executed with open('Example2.txt', 'r') as testwritefile: print(testwritefile.read())

This is line A
This is line B
This is line C

However, note that setting the mode to w overwrites all the existing data in the file.

In [8]:

with open('Example2.txt', 'w') as writefile:
    writefile.write("Overwrite\n")
with open('Example2.txt', 'r') as testwritefile:
    print(testwritefile.read())

with open('Example2.txt', 'w') as writefile: writefile.write("Overwrite\n") with open('Example2.txt', 'r') as testwritefile: print(testwritefile.read())

Overwrite

---

Appending Files

We can write to files without losing any of the existing data as follows by setting the mode argument to append: a. you can append a new line as follows:

In [9]:

# Write a new line to text file

with open('Example2.txt', 'a') as testwritefile:
    testwritefile.write("This is line C\n")
    testwritefile.write("This is line D\n")
    testwritefile.write("This is line E\n")

# Write a new line to text file with open('Example2.txt', 'a') as testwritefile: testwritefile.write("This is line C\n") testwritefile.write("This is line D\n") testwritefile.write("This is line E\n")

You can verify the file has changed by running the following cell:

In [10]:

# Verify if the new line is in the text file

with open('Example2.txt', 'r') as testwritefile:
    print(testwritefile.read())

# Verify if the new line is in the text file with open('Example2.txt', 'r') as testwritefile: print(testwritefile.read())

Overwrite
This is line C
This is line D
This is line E

---

Additional modes

It's fairly ineffecient to open the file in a or w and then reopening it in r to read any lines. Luckily we can access the file in the following modes:

• r+ : Reading and writing. Cannot truncate the file.
• w+ : Writing and reading. Truncates the file.
• a+ : Appending and Reading. Creates a new file, if none exists.

You dont have to dwell on the specifics of each mode for this lab.

Let's try out the a+ mode:

In [11]:

with open('Example2.txt', 'a+') as testwritefile:
    testwritefile.write("This is line E\n")
    print(testwritefile.read())

with open('Example2.txt', 'a+') as testwritefile: testwritefile.write("This is line E\n") print(testwritefile.read())

There were no errors but read() also did not output anything. This is because of our location in the file.

Most of the file methods we've looked at work in a certain location in the file. .write() writes at a certain location in the file. .read() reads at a certain location in the file and so on. You can think of this as moving your pointer around in the notepad to make changes at specific location.

Opening the file in w is akin to opening the .txt file, moving your cursor to the beginning of the text file, writing new text and deleting everything that follows. Whereas opening the file in a is similiar to opening the .txt file, moving your cursor to the very end and then adding the new pieces of text.
It is often very useful to know where the 'cursor' is in a file and be able to control it. The following methods allow us to do precisely this -

• .tell() - returns the current position in bytes
• .seek(offset,from) - changes the position by 'offset' bytes with respect to 'from'. From can take the value of 0,1,2 corresponding to beginning, relative to current position and end

Now lets revisit a+

In [12]:

with open('Example2.txt', 'a+') as testwritefile:
    print("Initial Location: {}".format(testwritefile.tell()))
    
    data = testwritefile.read()
    if (not data):  #empty strings return false in python
            print('Read nothing') 
    else: 
            print(testwritefile.read())
            
    testwritefile.seek(0,0) # move 0 bytes from beginning.
    
    print("\nNew Location : {}".format(testwritefile.tell()))
    data = testwritefile.read()
    if (not data): 
            print('Read nothing') 
    else: 
            print(data)
    
    print("Location after read: {}".format(testwritefile.tell()) )

with open('Example2.txt', 'a+') as testwritefile: print("Initial Location: {}".format(testwritefile.tell())) data = testwritefile.read() if (not data): #empty strings return false in python print('Read nothing') else: print(testwritefile.read()) testwritefile.seek(0,0) # move 0 bytes from beginning. print("\nNew Location : {}".format(testwritefile.tell())) data = testwritefile.read() if (not data): print('Read nothing') else: print(data) print("Location after read: {}".format(testwritefile.tell()) )

Initial Location: 70
Read nothing

New Location : 0
Overwrite
This is line C
This is line D
This is line E
This is line E

Location after read: 70

Finally, a note on the difference between w+ and r+. Both of these modes allow access to read and write methods, however, opening a file in w+ overwrites it and deletes all pre-existing data.
To work with a file on existing data, use r+ and a+. While using r+, it can be useful to add a .truncate() method at the end of your data. This will reduce the file to your data and delete everything that follows.
In the following code block, Run the code as it is first and then run it with the .truncate().

In [13]:

with open('Example2.txt', 'r+') as testwritefile:
    data = testwritefile.readlines()
    testwritefile.seek(0,0) #write at beginning of file
   
    testwritefile.write("Line 1" + "\n")
    testwritefile.write("Line 2" + "\n")
    testwritefile.write("Line 3" + "\n")
    testwritefile.write("finished\n")
    #Uncomment the line below
    # testwritefile.truncate()
    testwritefile.seek(0,0)
    print(testwritefile.read())
    

with open('Example2.txt', 'r+') as testwritefile: data = testwritefile.readlines() testwritefile.seek(0,0) #write at beginning of file testwritefile.write("Line 1" + "\n") testwritefile.write("Line 2" + "\n") testwritefile.write("Line 3" + "\n") testwritefile.write("finished\n") #Uncomment the line below # testwritefile.truncate() testwritefile.seek(0,0) print(testwritefile.read())

Line 1
Line 2
Line 3
finished
is line D
This is line E
This is line E

---

Copy a File

Let's copy the file Example2.txt to the file Example3.txt:

In [14]:

# Copy file to another

with open('Example2.txt','r') as readfile:
    with open('Example3.txt','w') as writefile:
          for line in readfile:
                writefile.write(line)

# Copy file to another with open('Example2.txt','r') as readfile: with open('Example3.txt','w') as writefile: for line in readfile: writefile.write(line)

We can read the file to see if everything works:

In [15]:

# Verify if the copy is successfully executed

with open('Example3.txt','r') as testwritefile:
    print(testwritefile.read())

# Verify if the copy is successfully executed with open('Example3.txt','r') as testwritefile: print(testwritefile.read())

Line 1
Line 2
Line 3
finished
is line D
This is line E
This is line E

After reading files, we can also write data into files and save them in different file formats like .txt, .csv, .xls (for excel files) etc. You will come across these in further examples

Now go to the directory to ensure the .txt file exists and contains the summary data that we wrote.

---

Exercise

Your local university's Raptors fan club maintains a register of its active members on a .txt document. Every month they update the file by removing the members who are not active. You have been tasked with automating this with your Python skills.
Given the file currentMem, Remove each member with a 'no' in their Active column. Keep track of each of the removed members and append them to the exMem file. Make sure that the format of the original files in preserved. (Hint: Do this by reading/writing whole lines and ensuring the header remains )
Run the code block below prior to starting the exercise. The skeleton code has been provided for you. Edit only the cleanFiles function.

In [3]:

#Run this prior to starting the exercise
from random import randint as rnd

memReg = 'members.txt'
exReg = 'inactive.txt'
fee =('yes','no')

def genFiles(current,old):
    with open(current,'w+') as writefile: 
        writefile.write('Membership No  Date Joined  Active  \n')
        data = "{:^13}  {:<11}  {:<6}\n"

        for rowno in range(20):
            date = str(rnd(2015,2020))+ '-' + str(rnd(1,12))+'-'+str(rnd(1,25))
            writefile.write(data.format(rnd(10000,99999),date,fee[rnd(0,1)]))


    with open(old,'w+') as writefile: 
        writefile.write('Membership No  Date Joined  Active  \n')
        data = "{:^13}  {:<11}  {:<6}\n"
        for rowno in range(3):
            date = str(rnd(2015,2020))+ '-' + str(rnd(1,12))+'-'+str(rnd(1,25))
            writefile.write(data.format(rnd(10000,99999),date,fee[1]))


genFiles(memReg,exReg)

#Run this prior to starting the exercise from random import randint as rnd memReg = 'members.txt' exReg = 'inactive.txt' fee =('yes','no') def genFiles(current,old): with open(current,'w+') as writefile: writefile.write('Membership No Date Joined Active \n') data = "{:^13} {:<11} {:<6}\n" for rowno in range(20): date = str(rnd(2015,2020))+ '-' + str(rnd(1,12))+'-'+str(rnd(1,25)) writefile.write(data.format(rnd(10000,99999),date,fee[rnd(0,1)])) with open(old,'w+') as writefile: writefile.write('Membership No Date Joined Active \n') data = "{:^13} {:<11} {:<6}\n" for rowno in range(3): date = str(rnd(2015,2020))+ '-' + str(rnd(1,12))+'-'+str(rnd(1,25)) writefile.write(data.format(rnd(10000,99999),date,fee[1])) genFiles(memReg,exReg)

Now that you've run the prerequisite code cell above, which prepared the files for this exercise, you are ready to move on to the implementation.

Exercise: Implement the cleanFiles function in the code cell below.¶

In [20]:

  
'''
The two arguments for this function are the files:
    - currentMem: File containing list of current members
    - exMem: File containing list of old members
    
    This function should remove all rows from currentMem containing 'no' 
    in the 'Active' column and appends them to exMem.
    '''
def cleanFiles(currentMem, exMem):
    # current_mem_l =[]
    # innactiv_mem_l =[]
    # TODO: Open the currentMem file as in r+ mode
    with open(currentMem,'r+') as curMemfile: 
        #TODO: Open the exMem file in a+ mode
        # f_header = curMemfile.read(0)
        # print(curMemfile.read(0))
        
        with open(exMem,'a+') as exMemfile: 
        #TODO: Read each member in the currentMem (1 member per row) file into a list.
        # Hint: Recall that the first line in the file is the header.
            curMemfile.seek(0, 0)
            for cur_mem in curMemfile.readlines():
                current_mem_l = cur_mem   
                # print('current_mem_l : ', current_mem_l)
        #TODO: iterate through the members and create a new list of the innactive members
        # Go to the beginning of the currentMem file
        # TODO: Iterate through the members list. 
        # If a member is inactive, add them to exMem, otherwise write them into currentMem
                # print('current_mem_l : ',current_mem_l)
                curMemfile.seek(0, 0)
                curMemfile.write(current_mem_l)
                if 'yes' in current_mem_l:
                    curMemfile.write(current_mem_l)
                elif 'no' in current_mem_l:
                    exMemfile.write(current_mem_l)
                
#                  print('current_mem_l : ', current_mem_l)
#                  print('innactiv_mem_l : ', innactiv_mem_l)
            curMemfile.truncate()

# The code below is to help you view the files.
# Do not modify this code for this exercise.
memReg = 'members.txt'
exReg = 'inactive.txt'
cleanFiles(memReg,exReg)


headers = "Membership No  Date Joined  Active  \n"
with open(memReg,'r') as readFile:
    print("Active Members: \n\n")
    print(readFile.read())
    
with open(exReg,'r') as readFile:
    print("Inactive Members: \n\n")
    print(readFile.read())
                
    

''' The two arguments for this function are the files: - currentMem: File containing list of current members - exMem: File containing list of old members This function should remove all rows from currentMem containing 'no' in the 'Active' column and appends them to exMem. ''' def cleanFiles(currentMem, exMem): # current_mem_l =[] # innactiv_mem_l =[] # TODO: Open the currentMem file as in r+ mode with open(currentMem,'r+') as curMemfile: #TODO: Open the exMem file in a+ mode # f_header = curMemfile.read(0) # print(curMemfile.read(0)) with open(exMem,'a+') as exMemfile: #TODO: Read each member in the currentMem (1 member per row) file into a list. # Hint: Recall that the first line in the file is the header. curMemfile.seek(0, 0) for cur_mem in curMemfile.readlines(): current_mem_l = cur_mem # print('current_mem_l : ', current_mem_l) #TODO: iterate through the members and create a new list of the innactive members # Go to the beginning of the currentMem file # TODO: Iterate through the members list. # If a member is inactive, add them to exMem, otherwise write them into currentMem # print('current_mem_l : ',current_mem_l) curMemfile.seek(0, 0) curMemfile.write(current_mem_l) if 'yes' in current_mem_l: curMemfile.write(current_mem_l) elif 'no' in current_mem_l: exMemfile.write(current_mem_l) # print('current_mem_l : ', current_mem_l) # print('innactiv_mem_l : ', innactiv_mem_l) curMemfile.truncate() # The code below is to help you view the files. # Do not modify this code for this exercise. memReg = 'members.txt' exReg = 'inactive.txt' cleanFiles(memReg,exReg) headers = "Membership No Date Joined Active \n" with open(memReg,'r') as readFile: print("Active Members: \n\n") print(readFile.read()) with open(exReg,'r') as readFile: print("Inactive Members: \n\n") print(readFile.read())

Active Members: 


    89883      2019-8-2     yes   
    89883      2019-8-2     yes   

Inactive Members: 


Membership No  Date Joined  Active  
    35891      2019-1-14    no    
    78796      2019-7-6     no    
    89400      2015-12-20   no    
    50735      2015-11-9    no    
    66207      2020-8-18    no    
    72862      2019-7-24    no    
    52291      2017-9-15    no    
    90674      2016-6-13    no    
    35806      2017-3-9     no    
    31439      2020-4-14    no    
    51042      2017-12-25   no    
    49408      2019-9-10    no    
    72284      2017-6-2     no    
    50735      2015-11-9    no    
    66207      2020-8-18    no    
    72862      2019-7-24    no    
    52291      2017-9-15    no    
    90674      2016-6-13    no    
    35806      2017-3-9     no    
    31439      2020-4-14    no    
    51042      2017-12-25   no    
    49408      2019-9-10    no    
    72284      2017-6-2     no    
    50735      2015-11-9    no    
    66207      2020-8-18    no    
    72862      2019-7-24    no    
    52291      2017-9-15    no    
    90674      2016-6-13    no    
    35806      2017-3-9     no    
    31439      2020-4-14    no    
    51042      2017-12-25   no    
    49408      2019-9-10    no    
    72284      2017-6-2     no    
    50735      2015-11-9    no    
    66207      2020-8-18    no    
    72862      2019-7-24    no    
    52291      2017-9-15    no    
    90674      2016-6-13    no    
    35806      2017-3-9     no    
    31439      2020-4-14    no    
    51042      2017-12-25   no    
    49408      2019-9-10    no    
    72284      2017-6-2     no    
    66207      2020-8-18    no    
    72862      2019-7-24    no    
    52291      2017-9-15    no    
    90674      2016-6-13    no    
    35806      2017-3-9     no    
    31439      2020-4-14    no    
    51042      2017-12-25   no    
    49408      2019-9-10    no    
    72284      2017-6-2     no    

The code cell below is to verify your solution. Please do not modify the code and run it to test your implementation of cleanFiles.

In [ ]:

def testMsg(passed):
    if passed:
       return 'Test Passed'
    else :
       return 'Test Failed'

testWrite = "testWrite.txt"
testAppend = "testAppend.txt" 
passed = True

genFiles(testWrite,testAppend)

with open(testWrite,'r') as file:
    ogWrite = file.readlines()

with open(testAppend,'r') as file:
    ogAppend = file.readlines()

try:
    cleanFiles(testWrite,testAppend)
except:
    print('Error')

with open(testWrite,'r') as file:
    clWrite = file.readlines()

with open(testAppend,'r') as file:
    clAppend = file.readlines()
        
# checking if total no of rows is same, including headers

if (len(ogWrite) + len(ogAppend) != len(clWrite) + len(clAppend)):
    print("The number of rows do not add up. Make sure your final files have the same header and format.")
    passed = False
    
for line in clWrite:
    if  'no' in line:
        passed = False
        print("Inactive members in file")
        break
    else:
        if line not in ogWrite:
            print("Data in file does not match original file")
            passed = False
print ("{}".format(testMsg(passed)))
    

def testMsg(passed): if passed: return 'Test Passed' else : return 'Test Failed' testWrite = "testWrite.txt" testAppend = "testAppend.txt" passed = True genFiles(testWrite,testAppend) with open(testWrite,'r') as file: ogWrite = file.readlines() with open(testAppend,'r') as file: ogAppend = file.readlines() try: cleanFiles(testWrite,testAppend) except: print('Error') with open(testWrite,'r') as file: clWrite = file.readlines() with open(testAppend,'r') as file: clAppend = file.readlines() # checking if total no of rows is same, including headers if (len(ogWrite) + len(ogAppend) != len(clWrite) + len(clAppend)): print("The number of rows do not add up. Make sure your final files have the same header and format.") passed = False for line in clWrite: if 'no' in line: passed = False print("Inactive members in file") break else: if line not in ogWrite: print("Data in file does not match original file") passed = False print ("{}".format(testMsg(passed)))

Click here for the solution

def cleanFiles(currentMem,exMem):
    with open(currentMem,'r+') as writeFile: 
        with open(exMem,'a+') as appendFile:
            #get the data
            writeFile.seek(0)
            members = writeFile.readlines()
            #remove header
            header = members[0]
            members.pop(0)
                
            inactive = [member for member in members if ('no' in member)]
            '''
            The above is the same as 

            for member in members:
            if 'no' in member:
                inactive.append(member)
            '''
            #go to the beginning of the write file
            writeFile.seek(0) 
            writeFile.write(header)
            for member in members:
                if (member in inactive):
                    appendFile.write(member)
                else:
                    writeFile.write(member)      
            writeFile.truncate()
                
memReg = 'members.txt'
exReg = 'inactive.txt'
cleanFiles(memReg,exReg)

# code to help you see the files

headers = "Membership No  Date Joined  Active  \n"

with open(memReg,'r') as readFile:
    print("Active Members: \n\n")
    print(readFile.read())
    
with open(exReg,'r') as readFile:
    print("Inactive Members: \n\n")
    print(readFile.read())
    

---

The last exercise!

Congratulations, you have completed this lesson and hands-on lab in Python.

---

Author¶

Joseph Santarcangelo

Other Contributors¶

Mavis Zhou

Change Log¶

Date (YYYY-MM-DD)	Version	Changed By	Change Description
2022-01-10	2.1	Malika	Removed the readme for GitShare
2020-10-16	1.3	Arjun Swani	Added exercise
2020-10-16	1.2	Arjun Swani	Added section additional file modes
2020-10-16	1.1	Arjun Swani	Made append a different section
2020-08-28	0.2	Lavanya	Moved lab to course repo in GitLab

---

© IBM Corporation 2020. All rights reserved.