Date formatting with mdates in Matplotlib - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the x-axis date format shown by this code?

This code plots dates on the x-axis using matplotlib and mdates. What date format will appear on the x-axis labels?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime

dates = [datetime.datetime(2024, 1, i+1) for i in range(5)]
values = [10, 20, 15, 25, 30]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
plt.show()

A Dates formatted as 'DD-MM-YYYY', e.g., '01-01-2024'

B Dates formatted as 'MM/DD/YYYY', e.g., '01/01/2024'

C Dates formatted as 'Month day, Year', e.g., 'January 1, 2024'

D Dates formatted as 'YYYY-MM-DD', e.g., '2024-01-01'

💡 Hint

Look at the string inside DateFormatter to see the date format pattern.

The DateFormatter is set with '%Y-%m-%d', which means year-month-day with dashes, so the dates appear like '2024-01-01'.

❓ data_output

intermediate

How many major ticks appear on the x-axis?

Given this code that plots 10 daily dates and sets the major locator to mdates.DayLocator(interval=3), how many major ticks will appear on the x-axis?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime

dates = [datetime.datetime(2024, 3, i+1) for i in range(10)]
values = range(10)

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_locator(mdates.DayLocator(interval=3))
plt.close(fig)  # Close plot to avoid display

A 4

B 3

C 5

D 10

💡 Hint

Count days from 1 to 10 with steps of 3: days 1,4,7,10.

DayLocator with interval=3 places ticks every 3 days starting from the first date, so ticks appear on days 1,4,7,10, totaling 4 ticks.

🔧 Debug

advanced

Why does this code raise a TypeError?

This code tries to format dates on the x-axis but raises a TypeError. What is the cause?

Matplotlib

import matplotlib.pyplot as plt
import matplotlib.dates as mdates
import datetime

dates = ['2024-01-01', '2024-01-02', '2024-01-03']
values = [1, 2, 3]

fig, ax = plt.subplots()
ax.plot(dates, values)
ax.xaxis.set_major_formatter(mdates.DateFormatter('%Y-%m-%d'))
plt.show()

A Dates are strings, but DateFormatter expects datetime objects, causing TypeError.

B The DateFormatter format string '%Y-%m-%d' is invalid and causes TypeError.

C The plot function cannot accept lists as x-axis values, causing TypeError.

D Missing plt.show() causes TypeError when rendering the plot.

💡 Hint

Check the type of the dates list elements.

DateFormatter expects datetime objects to format dates. Passing strings causes a TypeError because it cannot format string objects as dates.

❓ visualization

advanced

Which option correctly sets monthly ticks with abbreviated month names?

You want the x-axis to show ticks at the start of each month with labels like 'Jan', 'Feb', etc. Which code snippet achieves this?

A

ax.xaxis.set_major_locator(mdates.WeekdayLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))

B

ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))

C

ax.xaxis.set_major_locator(mdates.MonthLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%B'))

D

ax.xaxis.set_major_locator(mdates.DayLocator())
ax.xaxis.set_major_formatter(mdates.DateFormatter('%b'))

💡 Hint

Look for MonthLocator and '%b' for abbreviated month names.

MonthLocator sets ticks at month starts. DateFormatter with '%b' formats months as abbreviated names like 'Jan'.

🧠 Conceptual

expert

What happens if you set both major and minor locators with mdates but only format major ticks?

If you set ax.xaxis.set_major_locator(mdates.MonthLocator()) and ax.xaxis.set_minor_locator(mdates.WeekdayLocator()), but only set a major formatter with DateFormatter, what will the minor ticks show?

A Minor ticks cause an error because they lack a formatter.

B Minor ticks show the same labels as major ticks, causing label overlap.

C Minor ticks appear without labels because no formatter is set for them.

D Minor ticks are hidden automatically if no formatter is set.

💡 Hint

Think about how matplotlib handles tick labels when no formatter is assigned.

Minor ticks appear as marks but have no labels if no formatter is assigned. Only major ticks get labels from the major formatter.