Python Scripts and Values

Programming FoundationsLesson 2 of 860 minutesLab included

Learning objectives

By the end of this lesson

• Distinguish integers, strings, and Boolean values.
• Explain what a variable name refers to.
• Accept terminal input through sys.argv.
• Convert text to an integer and interpret common errors.

Values have types

Programs do not treat 12 and "12" as the same value. The first is an integer suitable for arithmetic. The second is a string containing two characters. A type describes what kind of value something is and which operations make sense for it.

Create values.py:

count = 12
label = "files"
complete = False

print(type(count), count)
print(type(label), label)
print(type(complete), complete)

Run it with python3 values.py. A variable is a name bound to a value; it is not a box permanently restricted to one kind of value. Python tracks the value’s type at runtime.

Change count to "12", predict what count + 3 will do, then try it. The resulting TypeError tells you the operation and incompatible types. Restore the integer afterward.

Input begins as text

Create arguments.py:

import sys

print("program:", sys.argv[0])
print("arguments:", sys.argv[1:])

Run:

python3 arguments.py alpha 27

sys.argv is a list of strings. Index zero identifies the script; later entries contain supplied arguments. Even 27 arrives as text.

Convert deliberately:

import sys

count = int(sys.argv[1])
print("next:", count + 1)

Try no argument, 12, and twelve. Record the IndexError and ValueError. Each failure identifies a different unsupported assumption.

Knowledge check

Question 1 of 2

Why does 12 + 3 work while "12" + 3 fails?

The values have different types and the operation is not defined for string plus integerQuotation marks make a number negativePython cannot add values from source files

Check answer

What this gives us as researchers

You can identify values entering a program, track their types, and recognize the assumptions introduced by conversion. Next we will follow those values through decisions and repeated work.