Debugging Your First Program
Next, we will write our first Python program.
From Instructions to Output
A first program becomes easier to understand when you follow it as a sequence of instructions. Python runs one statement at a time from top to bottom. A complete program can assign values to variables, modify those values, and display the current results.
What do you think happens?
Before tracing the program, predict the two numeric values that will be displayed and describe what will happen to the multi-line text.
Reveal answer
Answer: The numeric output is 5 followed by 6. The multi-line text is then displayed with its internal line break preserved.
The first assignment gives i the value 5. The later assignment evaluates i + 1 and stores 6 back in i. The final print displays a string whose line break is part of its value.
Tracing the Variable i
The equals sign in an assignment is an instruction that connects a variable name to a value. In i = 5, Python associates the name i with the value 5. This is not a mathematical equation. Assignment does not produce screen output by itself; output occurs when a print statement asks Python to display a value.
The statement i = i + 1 is read in two stages. Python first looks at the current value of i and evaluates the right side. When i contains 5, i + 1 produces 6. Python then assigns that new value back to i, replacing the old value. The variable therefore has a current value that depends on the statements that have already run.
Following i One Statement at a Time
Determine the value displayed by each print statement in this sequence.
Assignment: i = 5 connects the variable i to the value 5. Nothing is displayed yet.
First output: print i reads the current value of i, so it displays 5.
Modification: i = i + 1 evaluates to 6 and stores 6 in i.
Second output: The next print i reads the updated value, so it displays 6.
The numeric output appears in this order: 5, then 6.
A Complete First Program
i = 5 print i i = i + 1 print i s = '''First line Second line''' print s
5
6
First line
Second lineThe first print statement sees 5 because the increment has not happened yet. The second print statement sees 6 because the assignment i = i + 1 has already replaced the old value. The final print displays the complete string, including its internal line break.
Multi-Line Text Values
Triple quotes allow a string to span multiple lines. Everything between the opening triple quotes and the closing triple quotes belongs to the string value, including the line break in the middle. A single-line string uses single or double quotes and fits on one line; a triple-quoted string can preserve internal line breaks.
First line
Second lineMistakes in Sequential Programs
Treating i = i + 1 as a mathematical equation.
The equals sign performs assignment. Python evaluates the right side using the current value of i and then stores the result back in i.
Fix:
Trace it in two stages: if i is 5, evaluate i + 1 as 6, then assign 6 to i.Expecting an assignment statement to display a value.
Assignment connects a variable name to a value but does not ask Python to display anything.
Fix:
Use print i when you want to display the current value.Forgetting that statement order affects output.
Python executes from top to bottom, so the variable has already changed by the time the moved print statement runs.
Fix:
Record the variable's current value immediately before each print statement.Treating a multi-line string as if its line break disappears.
The internal line break is part of the string value and is preserved when the string is displayed.
Fix:
Expect the output to follow the line structure written between the triple quotes.
When debugging a short program, make a trace before changing code. Write down the current value of each variable after every assignment, then compare that trace with the actual output. If the output differs from your expectation, inspect the statements immediately before the unexpected value appears.
Trace Before You Run
Trace this program from top to bottom. Write the value of i immediately before each print statement, then write the exact output order. i = 5 print i i = i + 1 print i
Hints
- The first assignment gives i the value 5.
- The second assignment evaluates the current value of i plus 1.
- A print statement displays the value that exists at that point in the sequence.
Create a short program that assigns a two-line string to s with triple quotes and then displays s. Before running it, predict whether the output will appear on one line or two.
Hints
- Place the line break between the opening and closing triple quotes.
- Use print s after the assignment.
- The internal line break is part of the string value.
Key Takeaways
- Python executes statements from top to bottom, one statement at a time.
- Assignment connects a variable name to a value; a later assignment can replace the variable's current value.
- The statement i = i + 1 reads the current value, adds 1, and assigns the result back to i.
- The print statement displays the value a variable has at the moment print runs.
- Triple-quoted strings can contain multiple lines, and their internal line breaks are preserved when displayed.
Key Takeaways
- A complete Python program combines assignment, modification, and output in a top-to-bottom sequence.
- A variable does not have one permanent value during execution; its current value depends on the assignments that have already run.
- Use print to inspect a variable's current numeric or string value.
- Triple quotes preserve line breaks inside a multi-line string.
- To debug this kind of program, trace each variable after every statement and compare the trace with the displayed output.