Strings in depth

Most data that reaches a program arrives as text: a customer typing their name into a form, an order reference printed on an invoice, a line in a server log, a message from a support chat. Before you can clean, check or analyse any of it, you need a precise picture of what a string is and what you can and cannot do with one.

You have already met strings, len() and a few string methods. This lesson goes one level deeper: how Python stores a string as a sequence of characters, why you can never change a string in place, how to walk through it character by character, and how to write characters that are hard to type, such as a tab, a newline or a backslash.

A string is a sequence of characters

Every character in a string has a position called an index. The first character is at index 0. Negative indexes count from the end: -1 is the last character, -2 the one before it.

A slice text[start:stop] gives you a new string from start up to, but not including, stop. Leave out start to begin at the start; leave out stop to run to the end. A third number, the step, takes every n-th character, and a step of -1 walks backwards.

ref = "INV-2025-00417"

print(ref[0])        # first character
print(ref[-1])       # last character
print(ref[:3])       # the prefix
print(ref[4:8])      # the year
print(ref[-5:])      # the last five characters
print(ref[::2])      # every second character
print(ref[::-1])     # reversed
print(len(ref))

Two details are worth remembering:

• An index that is out of range raises IndexError, but a slice that is out of range never does. ref[50:] is simply "", the empty string.
• ref[-5:] reads as "the last five characters". It is the cleanest way to take a suffix of any length.

Strings are immutable

Immutable means "cannot be changed after it is created". Every string method and every slice gives you a new string; the original stays exactly as it was. If you try to assign to a position, Python refuses:

code = "sku-1042"
code[0] = "S"

To "change" a string you build a new one and, if you want, store it back under the same name. The old string is discarded.

A method such as upper() follows the same rule. Calling name.upper() on its own line does nothing useful: it creates an upper-case copy and throws it away. You must keep the result.

code = "sku-1042"
fixed = code[:3].upper() + code[3:]
print(code)    # unchanged
print(fixed)

name = "noor"
name.upper()           # result discarded
print(name)
name = name.upper()    # result kept
print(name)

Looping over characters and testing with in

A for loop over a string gives you one character at a time, from left to right. Combined with the in operator you can classify each one. in works two ways on strings:

• ch in "aeiou" checks whether one character is one of the listed characters.
• "refund" in message checks whether a whole substring appears anywhere inside a longer string.

message = "Order 5521 arrived damaged, please send a refund for 2 items"

digits = 0
for ch in message:
    if ch in "0123456789":
        digits += 1
print("Digits:", digits)

print("refund" in message)
print("Refund" in message)   # in is case-sensitive
print("" in message)          # the empty string is in every string

code = "SKU-48213-BLK"
print(code[4:9])
print(code[-3:])
print(code[::-1][:3])
print("482" in code, "blk" in code)

Escape sequences

Some characters cannot be typed directly inside quotes. An escape sequence is a backslash followed by a code that stands for one character:

Each escape counts as one character: len("a\tb") is 3.

report = "Item\tQty\nPens\t12\nPaper\t3"
print(report)
print(len("a\tb"))
print("She said \"thanks\" and left")
print('It\'s ready')
print("Caf\u00e9 order")

Raw strings

Backslashes become a problem when the text itself is full of them, such as Windows folder paths or (later in this track) regular expressions. In "C:\new\tables" Python sees \n and \t and quietly turns them into a newline and a tab.

A raw string has an r before the opening quote: r"C:\new\tables". Inside it, a backslash is an ordinary character and nothing is escaped. One restriction: a raw string cannot end with a single backslash, because \" still stops the quote from closing the string.

normal = "C:\new\tables"
raw = r"C:\new\tables"
print(normal)
print(raw)
print(len(normal), len(raw))
print(raw == "C:\\new\\tables")

count_vowels("Invoice approved")   # 7
count_vowels("RHYTHM")             # 0
count_vowels("")                   # 0

mask_reference("INV-2024-0042")   # "*********0042"
mask_reference("12345")           # "*2345"
mask_reference("AB12")            # "AB12"

report_path("march.csv")   # C:\shared\new_orders\totals\march.csv  (as printed)

capitalise_first("riyadh branch")       # "Riyadh branch"
capitalise_first("al-Rashid Trading")   # "Al-Rashid Trading"  (the R stays upper case)
capitalise_first("")                    # ""