Python String Methods Are Lying to You About What They Return

I have seen this bug in production code written by developers with three or more years of experience.

name = "  hello world  "
name.strip()
name.upper()
print(name)

What does this print?

Not "HELLO WORLD". Not "hello world". It prints " hello world " exactly as it started.

Both operations were thrown away.

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Why String Methods Never Modify In Place

Strings in Python are immutable. Once a string object exists, it cannot be changed. Every string method that appears to modify a string is actually returning a brand new string object.

The operation ran. The result was computed. But you did not capture it anywhere, so it was immediately garbage collected.

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The Correct Pattern

name = "  hello world  "
name = name.strip()
name = name.upper()
print(name)

Output: HELLO WORLD

Or in one line using method chaining:

name = "  hello world  ".strip().upper()
print(name)

Each method returns a new string. The next method in the chain is called on that new string. The original string is never involved after the first call.

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Tracing Method Chaining

result = "  Python is Great  ".strip().lower().replace("is", "was").split()
print(result)

Trace each step:

Step 1: " Python is Great ".strip() returns "Python is Great"
Step 2: "Python is Great".lower() returns "python is great"
Step 3: "python is great".replace("is", "was") returns "python was great"
Step 4: "python was great".split() returns ["python", "was", "great"]

Output: ['python', 'was', 'great']

The original string " Python is Great " is untouched throughout. Four new string objects were created and three were immediately discarded.

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The Interview Problem

def process(text):
    text.strip()
    text = text.lower()
    text.replace(" ", "_")
    return text

result = process("  Hello World  ")
print(result)

Output: " hello world "

Line by line:

• text.strip() runs but the result is discarded. text is still " Hello World ".
• text = text.lower() runs and the result IS captured. text is now " hello world ".
• text.replace(" ", "_") runs but the result is discarded. text is still " hello world ".
• The function returns " hello world ". The strip was wasted and the replace was wasted. Only the lower() result was kept because only that line used assignment to capture the return value.

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Five Methods That Beginners Forget Return New Strings

s = "hello"

s.upper()        # returns "HELLO"  -- s unchanged
s.replace("h", "H")  # returns "Hello" -- s unchanged
s.strip()        # returns "hello"  -- s unchanged (no whitespace)
s.center(11)     # returns "   hello   " -- s unchanged
s.encode()       # returns b"hello" -- s unchanged

print(s)         # still "hello"

Compare to list methods that modify in place:

lst = [3, 1, 2]
lst.sort()       # modifies lst in place, returns None
lst.append(4)    # modifies lst in place, returns None
print(lst)       # [1, 2, 3, 4]

Lists are mutable so their methods modify in place. Strings are immutable so their methods return new objects. This asymmetry is the root of most string-related bugs.

Practice output prediction problems at PyCodeIt.

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