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Code Crew G6+ / Week 02 / Thursday
4/6
Week 02 · Variables and input

Thursday

Neat messages with f-strings
// Programs that remember and ask
⏱ about 30 min

Thursday: Neat messages with f-strings

Rocket's status report is a tangle of commas and quote marks. One line has nine pieces glued together, and the temperature shows eleven digits after the decimal point.

"Nobody can read this," he admits. "Not even me."

Raven squints at the screen. "What do you notice?" she asks. "The message is mostly words. The values are just a few small holes in the sentence."

Nova draws a sentence in the air with three glowing gaps. "Would you like a hint?" she asks. "Python has a string that fills its own gaps. You put an f in front and the values in curly brackets."

Rocket rewrites the longest line, and the report suddenly looks like a real sentence.

f-strings

An f-string has the letter f right before the opening quote. Inside it, anything in curly brackets { } is worked out and dropped into the text.

You can put a variable, or even a calculation, inside the brackets. Add :.1f after a number to show one digit after the decimal point, or :.2f for two.

# status.py
probe = "Probe 7"
battery = 87
temp = 21.456
print(f"{probe} battery: {battery}%")
print(f"Temperature: {temp:.1f} degrees")
print(f"Next check in {battery // 10} minutes")
Probe 7 battery: 87%
Temperature: 21.5 degrees
Next check in 8 minutes

Without the f, Python would show the curly brackets and the names inside them as plain text. Forgetting the f is a very common slip.

tanks = 6. f"{tanks} tanks ready" shows: ____ tanks ready
f"{3 * 4} hours" shows: ____ hours
x = 2 / 3. f"{x:.2f}" shows:
SPOT THE SLIP
  • Read the question.
  • Tap your answer.
fuel = 50. What does print("Fuel: {fuel}") show?
At your computer
1. Type status.py in a new file, save it and run it.
2. Change temp to 19.98 and run again. What does :.1f show now?
3. Delete the f before one of the strings. Run it and see what changes.
4. Put the f back. Then add a line that uses input() to ask for a probe name and shows it in an f-string.

Practice: more f-string tricks

Add a comma after the colon to put commas in big numbers, like {distance:,}. You can combine tricks too: {speed:,.1f} adds commas and keeps one decimal.

Remember that the work happens inside the curly brackets, so you can do math there.

# probe_report.py
probe = "Probe 9"
distance = 12500
speed = 3.14159
print(f"{probe} has gone {distance:,} km")
print(f"Speed: {speed:.2f} km per second")
print(f"Half way: {distance // 2:,} km")
Probe 9 has gone 12,500 km
Speed: 3.14 km per second
Half way: 6,250 km
READ THE F-STRINGS
  • Read the question.
  • Tap your answer.
What does f"{1500000:,}" show?
What does f"{2.678:.1f}" show?
Which line shows Crew: 3 when crew = 3?
tanks = 4. f"{tanks * 2} tanks" shows: ____ tanks
f"{10 / 4:.1f}" shows: ____
f"{20000:,}" shows: ____
StatementTrue or false?
f"{5 + 5}" shows 10.?
"{5 + 5}" with no f shows 10.?
:.2f shows two digits after the decimal point.?
The f goes after the closing quote mark.?
WHY THIS EXERCISEThe f before the opening quote makes Python fill the brackets. Without it, the brackets stay as plain text.
Try it
1. Type probe_report.py, save it and run it.
2. Change distance to 987654 and run it again. Check the commas.
3. Change :.2f to :.4f and see how many digits appear.
4. Add a line that shows the distance in meters, using {distance * 1000:,}.

f-strings make your output clear and friendly. Tomorrow you will put everything together in a trip time calculator.

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