Understanding the Performance Print Line in Python (Step-by-Step)

[Rajeev Bagra]

🔹 Introduction

When analyzing how fast a function runs, you may see code like this:

import time

L_N = [1]
for i in range(7):
L_N.append(L_N[-1] * 10)

for N in L_N:
t = time.perf_counter()
km = convert_to_km(N)
dt = time.perf_counter() – t
print(f”convert_to_km({N}) took {dt} seconds ({1/dt}/sec)”)

This code is used to measure performance of a function for different input sizes.

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🔹 Context: What is happening here?

1. Creating input sizes

L_N = [1]
for i in range(7):
L_N.append(L_N[-1] * 10)

👉 This generates:

[1, 10, 100, 1000, …]

✔ Each input is 10× larger than the previous

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1. Looping through inputs

for N in L_N:

👉 We test the function with different sizes

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1. Measuring time

t = time.perf_counter()
km = convert_to_km(N)
dt = time.perf_counter() – t

👉 Steps:

• Start timer → “t”
• Run function → “convert_to_km(N)”
• Compute time taken → “dt”

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🔹 The Key Line (Focus)

print(f”convert_to_km({N}) took {dt} seconds ({1/dt}/sec)”)

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🔹 Step-by-Step Breakdown

✅ 1. “f”…”” → f-string

👉 Allows inserting values using “{}”

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✅ 2. “convert_to_km({N})”

• “”convert_to_km(“” → text
• “{N}” → actual value

👉 Example:

convert_to_km(100)

✔ This is just display text, NOT a function call

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✅ 3. “took {dt} seconds”

• “{dt}” = execution time

👉 Example:

took 0.002 seconds

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✅ 4. “({1/dt}/sec)”

👉 This calculates speed

• “1/dt” = number of operations per second

Example:

dt = 0.002
1 / dt = 500

👉 Output:

(500/sec)

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🔹 Final Output Example

If:

N = 100
dt = 0.002

👉 Output:

convert_to_km(100) took 0.002 seconds (500/sec)

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🔹 Important Concept

👉 This line does NOT execute the function

The function is already executed here:

km = convert_to_km(N)

👉 The print line only reports results

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🔹 Why This is Useful

This pattern helps you:

• Compare performance across inputs
• Understand scaling (time complexity)
• See both:
• ⏱ Time taken (“dt”)
• ⚡ Speed (“1/dt”)

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🔹 Mental Model

👉 “For input size N:

• how long did it take?
• how fast is it?”

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🔹 One-Line Summary

👉 This print line converts raw timing into a human-readable performance report

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🔹 Bonus Insight

• Small “dt” → faster function
• Large “1/dt” → higher throughput

👉 Both together give a complete picture

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