How does the %s placeholder work in Python format strings?

Understanding the Meaning of %s in Python Format Strings

In Python, format strings use specific formatting tokens to insert values into strings, and one of these tokens is the placeholder %s which stands for "string substitution." It allows for the insertion of a string into the format string.

This placeholder is used in conjunction with the % operator, followed by the value to be inserted. For instance, if we have a variable called "name" that contains a string, we can insert it into a format string as follows:

<code class="python">"Hello, %s" % name</code>

This will result in the string "Hello, John" if the value of "name" is "John."

In the provided code snippet:

<code class="python">if len(sys.argv) < 2:
    sys.exit('Usage: %s database-name' % sys.argv[0])</code>

The %s placeholder is used to dynamically insert the program's name (sys.argv[0]) into the error message. This provides a more informative error message to the user, clearly stating the correct usage of the program.

Similarly, in the following line:

<code class="python">if not os.path.exists(sys.argv[1]):
    sys.exit('ERROR: Database %s was not found!' % sys.argv[1])</code>

The %s placeholder inserts the specified database name (sys.argv[1]) into the error message, indicating which database could not be found.

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