File

• In this lecture, we'll learn how to treat the files in Python.
• To access the files from Python code, you should use open function. When opening a file, you need to specify the mode.

• You can choose text or binary mode after the r, w, a character by specifying t or b. Text mode is used by default.

open

• You need to open the file anyway.

  f = open("filename.txt", "r")  # read
  f = open("filename.txt", "w")  # write
  f = open("filename.txt", "a")  # append
  

read

• To read the file, read function can be used.

  f = open("test.txt", "r")
  lines = f.read()
  

• However, read gives the whole file in one string, so not very useful for the file with multiple lines (which is usual).
• Instead, use readlines

  f = open("test.txt", "r")
  lines = f.readlines()
  

• This will give the list of strings contained in the file.

write

• You can write to file with write function.

  f = open("test.txt", "w")
  f.write("The first comment.\n")
  f.close()
  

• Note that w mode will overwrite the file. If you want append to a file, use the append mode a.

close

• After accessing the file, you should close the file.

  f = open("filename.txt", "w")
  f.close()
  

with

• You can use with statement to open the file, and in this way, you can omit the closing procedure of the file.

  with open("test.txt", "w") as f:
      f.write("Some string\n")
  

• reading text file

  with open("textfile.txt", "r") as f:
      for line in f:
          line = line.strip()
          print(line)
  

• strip is the function to remove change-line charactors.

Exercise: Writing and Reading from a File

1. Create a new text file and write some content to it using Python.
2. Read the content from the file you just created.

answer