Lecture 13


string formatting

MCS 260 Fall 2021
Emily Dumas


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A file is a named, ordered collection of data, usually in persistent storage (disks, flash, etc.).

Files are stored in a hierarchy of directories.

The OS provides functions that programs can use to access files, handling the lower-level details itself.

Basic file operations

  • open: Request access to a file (by name)
  • read: Get data from an open file
  • write: Add or change data in an open file
  • close: Relinquish access to an open file

The OS keeps track of a file offset, the place in the file where the next read or write operation will happen. This can also be moved with an operation called seek.

Bytes or strings?

Two ways to access files:

  • binary format: Read and write bytes (as the OS itself does)
  • text format: Read and write strings. Python translates to and from bytes using an encoding.

We'll only cover text format file operations for now.


An encoding is a way of turning a sequence of unicode characters into a sequence of bytes.

UTF-8, ISO-8859-1, and CP-1252 are examples of encodings.

E.g. translating "AdiĆ³s" into bytes:

  • 0x41 0x64 0x69 0xc3 0xb3 0x73 in UTF-8
  • 0x41 0x64 0x69 0xf3 0x73 in ISO-8859-1

We will use UTF-8 exclusively.

Files in Python

open(filename,[mode],[encoding=...]) opens a file and returns an object representing it.

Methods of the file object allow you to read or write.

        """Write a string to a file"""
        fout = open("out.txt","w",encoding="UTF-8")  # w means write
        fout.write("Hello world")
        fout.close() # Done with this file (OS does cleanup)

        fin = open("out.txt","r",encoding="UTF-8")   # r means read only (default)
        s = fin.read() # Get entire file contents
        print("Contents of file:",s)

The files you read/write this way can have any name you like; they don't need to end in ".txt".


  • "r" - The default. Allows reading. File must exist.
  • "w" - Deletes the file if it exists, creates it if not. Allows writing.
  • "a" - Place offset at the end of the file if it exists. Allows writing (i.e. "appending").
  • "r+" - Offset at beginning if file exists. Allows reading and writing.
  • "w+" - Deletes the file if it exists, creates it if not. Allows reading and writing.

Reading lines

Often you want to process one line at a time. File objects are iterable, giving the lines. E.g. nl.py

        """Number the lines of a file specified on command line"""
        import sys
        fin = open(sys.argv[1],"r",encoding="UTF-8")
        n = 0
        for line in fin:
            n = n+1
            print(n,line,end="") # line usually has \n at the end

Sample output:

$ python nl.py nl.py
1 """Number the lines of a file specified on command line"""
2 import sys

Important: file.write() is not like print(). It doesn't add a newline, and it doesn't accept multiple arguments to print.

        pet_type = "ducks"
        print("I have",21,pet_type) # OK
        fout.write("I have",21,pet_type) # FAILS

Must prepare a single string to write. The usual way is to use str.format():

        pet_type = "ducks"
        fout.write("I have {} {}\n".format(21,pet_type)) # ok

String formatting

str.format() has many features to create a string based on a template and some values. In the string, placeholders ({} or {...}) are replaced by arguments of str.format().

>>> "{1} taught {0}".format("MCS 260","Dumas")  # give indices
'Dumas taught MCS 260'
>>> for x in range(98,101):
...     print("{:4}".format(x))  # specify width
>>> "{:04}".format(42)  # pad to width with zeros

>>> "{:8.2f}".format(42)  # f = float, width 8, 2 digits after .
'   42.00'
>>> "{:8x}".format(42)    # x = hex int, width 8
'      2a'
>>> "{:8d}".format(42)    # d = decimal int, width 8
'      42'
>>> "{:.2f}".format(13+2j) # f allows complex; no total width

The general placeholder syntax is {w:ot} where w specifies which argument, o is a set of options, and t is the type.

str.format() has a lot of features we didn't discuss today.


Revision history

  • 2021-09-22 Initial publication