🔹 Introduction

When learning Python, you’ll often see code like:

print("convert_to_km(" + str(N) + ") took " + str(dt))

and later:

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

To truly understand the second (modern) version, you must first understand the first (basic) one.

This post builds your understanding step-by-step.

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🔹 Part 1: String Concatenation (The Foundation)

✅ The Code

print("convert_to_km(" + str(N) + ") took " + str(dt))

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🔹 What is happening?

You are joining multiple pieces of text into one string using +.

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🔹 Break it down

Part	Meaning
"convert_to_km("	plain text
str(N)	convert number → text
") took "	plain text
str(dt)	convert number → text

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🔹 Example

N = 100
dt = 0.002

Becomes:

"convert_to_km(" + "100" + ") took " + "0.002"

👉 Final output:

convert_to_km(100) took 0.002

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🔹 Why str() is required

"hello " + 5   ❌ ERROR

👉 Python does NOT mix string + number

✔ Fix:

"hello " + str(5)

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🔹 Key Idea

👉 + joins strings
👉 str() converts values to strings

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🔹 Part 2: Enter f-Strings (Modern Approach)

✅ The Code

print(f"convert_to_km({N}) took {dt} seconds")

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🔹 What changed?

Instead of manually joining:

• Python automatically inserts values using {}

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🔹 Meaning of {}

👉 {} = “evaluate this and insert the result”

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🔹 Example

N = 100
print(f"Value is {N}")

👉 Output:

Value is 100

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🔹 You can even calculate

dt = 0.002
print(f"{1/dt} per second")

👉 Output:

500 per second

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🔹 Part 3: Why {N} and not just N?

Without {}:

print("convert_to_km(N)")

👉 Output:

convert_to_km(N)

❌ Just text

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With {}:

print(f"convert_to_km({N})")

👉 Output:

convert_to_km(100)

✔ Value inserted

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🔹 Part 4: Is convert_to_km a Function Here?

Look carefully:

print(f"convert_to_km({N})")

👉 Important:

• "convert_to_km(" → text
• {N} → evaluated
• ")" → text

❌ The function is NOT being called here

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🔹 When is the function actually called?

km = convert_to_km(N)

👉 This executes the function

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🔹 Comparison

Code	What happens
convert_to_km(N)	✅ function runs
"convert_to_km(N)"	❌ just text
f"convert_to_km({N})"	✅ text + value

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🔹 Part 5: Can f-Strings Call Functions?

Yes:

print(f"{convert_to_km(N)}")

👉 This:

• Executes function
• Inserts result

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🔹 Part 6: Understanding the Full Performance Line

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

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🔹 Breakdown

Part	Meaning
convert_to_km({N})	shows input
{dt}	time taken
{1/dt}	speed (per second)

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🔹 Example Output

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

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🔹 Final Mental Model

👉 Inside quotes → text
👉 Inside {} → Python evaluates
👉 Outside quotes → real execution

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🔹 Conclusion

You’ve now learned the progression:

1. String Concatenation
   • + joins strings
   • str() converts values
2. f-Strings
   • {} inserts values automatically
   • Cleaner and more powerful
3. Critical Distinction
   • Writing a function name ≠ calling it

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🔹 One-Line Summary

👉 f-strings = clean way to combine text + variables + expressions

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