Python 3.6 Read File With While Not Eof

Python Read File Line past Line

In Python, you can read all the lines in a file using different methods. These include readlines (), readline () and context managers. You can read all the text in the files afterward opening them and perform other operations.

1. readlines (): read all lines in a file at one time
2. readline () with for loop
3. readline () with while loop
4. Context managers

1. Readlines (): Read All Lines in a File at Once

The readlines () method is the nearly pop method for reading all the lines of the file at one time. This method reads the file until EOF (End of file), which means it reads from the get-go line until the last line.  When yous apply this method on a file to read its lines, information technology returns a list. This list contains all the lines of the file. Hither, each line inside the list is a list object.

The syntax of the method is as follows:

file.readlines (num_line)

Here, the num_line parameter specifies the number of lines or number of bytes that take to be read from the file. But this parameter is optional. It takes an integer value. If the returned bytes exceed the number specified in the hint, the method volition stop returning lines. The default value for this method is -1, which ways all the lines inside the file volition be returned.

For instance,

          # Python 3 Code # Python Plan to read file line past line  # Open file mf = open("myfile.txt", "r+") print("File to read: ", mf.name)  # Read all lines in the file file_content = mf.readlines()  # Impress all lines in file print("Read Line: ", file_content)  # Close opened file mf.close()        

Output:

          File to read:  myfile.txt Read Line:  ['Line one\northward', 'Line two\n', 'Line three\north', 'Line four\n', 'Line five']        

The readlines () method is a very skilful option for reading all the contents of a small file. Simply you might face bug while working with big files.

ii. Using "While" Statement

At present we will look at how you can read the lines of a file ane by one using a while argument. If you lot are working with a large file with a lot of text, it is all-time to use the readline () method. This method fetches the lines one by one instead of retrieving all the text at one get.

Example,

          # Python iii Code # Python Plan to read file line by line # Using while statement  # Open file mf = open("myfile.txt", "r+") print("File to read: ", mf.proper name)  # Read single line in file file_line = mf.readline()  # use the readline() method to read further. # If the file is non empty keep reading one line # at a time, till the file is empty  while file_line:     print(file_line)     # apply realine() to read next line     file_line = mf.readline()  # Shut opened file mf.close()        

Output:

          Line one Line ii Line three Line four Line five        

Another example of the while argument is:

          # Python iii Lawmaking # Python Program to read file line by line # Using while statement  # Open file mf = open("myfile.txt", "r+") impress("File to read: ", mf.name)  while Truthful:     # Read single line in file     file_line = mf.readline()     print(file_line)     if not file_line:         break  # Close opened file mf.close()        

Output:

          Line one Line two Line three Line four Line five        

In the above code, the while statement checks for a Boolean value to be True. The readline () method reads the text line past line. When information technology reaches the stop of the file, the execution of the while loop stops.

3. Using "for" Loop

You can read the lines of a file in Python using a for loop. Use the following code for it:

          # Python 3 Code # Python Program to read file line by line # Using while argument  # Open file mf = open("myfile.txt", "r+") print("File to read: ", mf.proper noun)  for file_line in mf:     # Print single line     print(file_line)  # Close opened file mf.shut()        

Output:

          Line one Line two Line three Line 4 Line five        

Here, mf is the file handler that is used for opening the file chosen myfile.txt. A for loop is executed on every line in the text file. file_line  the variable used for iterating through the loop.

iv. Using Context Manager

If you lot are opening a file for some operation, y'all need to close it too. In case you don't shut it, it volition automatically exist closed when the part you are using to handle it completes execution. This happens when the last reference of the file handler is destroyed. But there may be a situation where the programs are long and the office will not complete execution soon.

In that case, you have to make use of a context managing director in Python. This manager will perform all the tasks such as closing files for yous - even if you lot forget to do it.

Take a look at this program,

          # Python 3 Lawmaking # Python Program to read file line past line # Using Context Manager List_file_lines = ["Apple tree", "Orange", "Banana", "Mango"]  # Define function to create file def create_file():     with open up ("testfile.txt", "w") as wf:         for line in List_file_lines:             # Write all lines             wf.write(line)             wf.write("\n")  # Define part to read files def read_File():     rf = open ("testfile.txt", "r")      # Read file lines     out = [] # listing to save lines     with open ("testfile.txt", "r") as rf:         # Read lines using for loop         for line in rf:             # All lines and strip last line which is newline             out.append(line.strip())     return out  # Define master part to create and read file def main():          # Create test file     create_file()          # Read lines from testfile.txt     outList = read_File()          # Iterate over the lines     for line in outList:         impress(line.strip())      # Run postal service function  if __name__ == "__main__":     main()        

Output:

          Apple tree Orange Banana Mango        

Here the with clause depicts the concept of context managers. This keeps track when you are opening the file and closes it as soon as the function ends.
Another example of context managers along with a while loop is as follows:

          # Python iii Code # Python plan to read file line by line # Using context manager & while Loop  with open ("myfile.txt", "r") as rf:     # Read each line of a file in the loop     for line in rf:         # Print every line in a file except the last line which is newline \due north         print(line.strip())                  

Output:

          Line 1 Line two Line 3 Line four Line five        

Conclusion

The method y'all will use for reading the lines of a file will depend upon the type of file you take. If y'all have a small file with a few lines of text, readlines () method is the all-time way to get. Using a for loop is as well practiced for small-scale files. For larger files, this method is not very retention efficient. Then, in that instance, you can use the while statement along with with the readline () method.

Context managers will make certain that you can read all lines of a file without worrying nearly closing the file handler.

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