String manipulation

Enables replacing occurrences of a substring within a string.

Data cleaning: normalising text fields by substituting legacy tokens Log processing: masking sensitive keywords in log lines User input handling: converting shorthand commands to full forms

Python scripts, data pipelines, web applications, command-line utilities

string.replace(old_substring, new_substring)

... .replace(..., ...)

string: str, old_substring: str, new_substring: str

• str.lower()
• str.strip()
• re.sub()

• Use re.sub() for regex‑based replacements – more flexible but slower Assign the result to a new variable versus reassigning the original – strings are immutable

{"cause":"Assuming replace mutates the original string","consequence":"The original variable remains unchanged, leading to unexpected output"} {"cause":"Forgetting to assign the returned value","consequence":"The transformed string is discarded, so no change is observed"} {"cause":"Calling replace on a non‑string object","consequence":"Raises a TypeError at runtime"}

str.replace (literal substitution) vs re.sub (regex substitution) str.replace vs str.translate (character‑wise replacement)

Coming from JavaScript: using string.replace without a global flag replaces only the first occurrence → Python's replace replaces all occurrences unless a count is given

• str.replace
• str.lower
• str.strip
• re.sub

Do not use when pattern‑based replacement is required; prefer re.sub for regular expressions Performance: creates a new string each call; large texts may increase memory usage Edge case: replacing an empty old_substring inserts the new substring between every character

Allows quick sanitisation and normalisation of textual data, reducing bugs and security issues in production pipelines.

Replace is like a find‑and‑swap sticker you press onto a sheet of paper, instantly changing every matching word.

Strings are immutable in Python; replace returns a new string that must be stored or used directly.

After mastering str.replace, progress to using re.sub for regex-based substitutions or str.translate for character-level translations.