Variables, Strings and Numbers

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A service response is text. Order number, status code, amount, state — all of it arrives as pieces of text that have to be stored somewhere, combined and compared. That is what variables are for: a name with a value attached.

A variable is a name for a value

live example

status = 200
order_id = "ord-042"

Name on the left, value on the right, an equals sign between them. It is not the "equals" of mathematics but "put the right side into the left". After that the name gives you the value:

print(order_id)      # prints ord-042

The name can be reassigned — that is what makes it a variable:

live example

status = 200
status = 404
print(status)        # 404

Numbers and strings are different things

This is what everyone trips over first:

live example

code = "200"        # a string: three characters of text
code = 200          # a number: you can do arithmetic with it

The quotes decide everything. For a person there is no difference, for a program there is a huge one:

live example

print("200" + "1")     # 2001 — strings are glued together
print(200 + 1)         # 201 — numbers are added

Mix them and the program stops with TypeError: can only concatenate str (not "int") to str. In plain words: a string can only be glued to a string. This is a common situation: everything arrives from a service as text, and arithmetic needs numbers.

String to number:

live example

total = int("4990")        # 4990, now a number
price = float("99.90")     # 99.9

And back:

live example

text = str(4990)           # "4990"

int() on the string "four" fails — and rightly so: there is nothing to convert.

Strings: the daily material

A string goes in quotes — single or double, it makes no difference.

Length:

live example

print(len("ord-042"))     # 7

Substitution — the f-string, the convenient way. Put an f before the quotes and write the expression in curly braces:

live example

order_id = "ord-042"
total = 4990
print(f"Order {order_id} for {total} RUB")

Searching inside a string — the in operator:

live example

response = "Order not found"
print("not found" in response)    # True

The result is True or False. Those two values carry every check you will ever write.

Useful string operations:

live example

"  ord-042  ".strip()        # "ord-042" — trim spaces
"ORD-042".lower()            # "ord-042"
"ord-042".startswith("ord")  # True
"ord-042".split("-")         # ["ord", "042"]
"ord-042".replace("ord", "x")  # "x-042"

strip() and lower() save you when comparing data from different sources: the export says "Anna ", the database says "anna", and to the eye it is the same thing — to the program it is not.

Common beginner mistakes

Unclosed quotes — print("hello). A SyntaxError with the line number; check quote and bracket pairs.

Typo in a name — you defined order_id and used order_di. NameError: name 'order_di' is not defined, that is, "I do not know that name".

Compared a number with a string — 200 == "200" is False. Not an error but a silent "did not match": the program does not crash, the test just behaves strangely. Fix it by converting to one type.

What to solve

In short

• A variable is a name for a value; = stores, it does not compare.
• "200" and 200 are different: text from a service must be converted with int() or float() before arithmetic.
• An f-string (f"Order {order_id}") is the normal way to build a message.
• strip() and lower() bring data from different sources to a comparable form.
• in answers True or False — every check starts there.

What to read next

• Comparisons and conditions — how a program makes a decision.
• Lists and dicts — what JSON looks like from the inside.