[Python] How to Repeatedly Generate and Initialize List Elements

In Python, using the * operator (multiplication operator) is the most concise and “Pythonic” way to create a list with specific initial values or to generate a repeating array pattern.

In this article, I will explain how to efficiently expand and initialize lists without using for loops.

Table of Contents

1. The Problem to Solve
2. Implementation Example: Generating Weekly Shift Patterns
3. Source Code
4. Execution Result
5. Explanation

---

The Problem to Solve

There are often situations in list initialization where you want to “set all elements to 0” or “repeat a specific pattern (A, B, C) N times.” Writing this with a loop makes the code verbose, so we utilize the operator to write it in a single line.

---

Implementation Example: Generating Weekly Shift Patterns

Here, we assume a scenario where we define a basic daily shift pattern (Early, Late, Night) and repeat it for one week (7 days) to create a large list.

Source Code

# Basic shift configuration for one day
# Defined as a list of strings
daily_pattern = ["Early", "Late", "Night"]

# Batch generate the schedule slots for one week (7 days)
# By doing [List] * [Integer], the contents of the list are concatenated that many times
weekly_schedule = daily_pattern * 7

# Check the result
print(f"Total shift slots: {len(weekly_schedule)}")
print("--- Generated List ---")
print(weekly_schedule)

Execution Result

Total shift slots: 21
--- Generated List ---
['Early', 'Late', 'Night', 'Early', 'Late', 'Night', 'Early', 'Late', 'Night', 'Early', 'Late', 'Night', 'Early', 'Late', 'Night', 'Early', 'Late', 'Night', 'Early', 'Late', 'Night']

---

Explanation

List Multiplication

Multiplying a list by an integer N creates a new list where the elements of the original list are repeated N times.

• Single Element Repetition:[0] * 5 → [0, 0, 0, 0, 0]
  • This is frequently used for array initialization (such as zero-padding).
• Pattern Repetition:["A", "B"] * 3 → ["A", "B", "A", "B", "A", "B"]
  • This is effective when repeating a sequence itself, as in this example.

Important Note: Reference Copy

The elements created by this method are “Shallow Copies.”

This is not a problem for immutable objects like numbers or strings. However, if you put a list inside a list (e.g., [[]] * 3), be careful because all internal lists will refer to the same object. For initializing multi-dimensional arrays, using list comprehension is recommended.