Working with files

File (file descriptor) in Python is object (as everything else)

• Opening file: f = open(<path>[, mode="<modificator>"][, encoding="utf-8"])

• Modificators:

  • r – read only (default method)

  • r+ – read and write

  • w – write (with flushing) (> in shell)

  • a – appending (>> in shell)

  • b – binary mode

Basic operations

• Reading:

  • f.read(N) – read N bytes (into string)

  • f.readline() – read one line (with separator)

  • f.readlines() – read all file in a list

• f.close() – closing file (Python closes it on exit)

• Writing:

  • f.write(text) – write string to file (w/o closing)

  • f.writelines(lines) – write list of strings to file

  • f.flush() – sync buffer to disk (w/o closing file)

Changing position:

From Python 3.2: In text files (those opened without a "b" in the mode string), only seeks relative to the beginning of the file are allowed (the exception being seeking to the very file end with seek(0, 2)).

• f.tell() – returns the file’s current position

• f.seek(N, [whence=0]) – shift N bytes from the beginning where whence (direction) can be:

  • 0 (from beginning)

  • 1 (from current position)

  • 2 (from the end)

Example:

Here we are using so-called "context managers" - via *with as" keywords. This allows us to not worry about closing the file descriptor after block of with-as code.

🪄 Code:

filename = "/tmp/1.txt"
# filename = r"D:\tmp.txt"

with open(filename, "w") as f:
    f.write("Test string!!!\n")
    
with open(filename) as f:
    print(f.read())
    
print(f.closed)

📟 Output:

Test string!!!

True

🪄 Code:

%ls -la /tmp/1.txt

-rw-r--r-- 1 jovyan users 15 Jun 20 11:52 /tmp/1.txt