TypeError: can only concatenate list (not "str") to list

What this error means

You used + between a list and a string. For lists, + means 'join two lists into a new list' — both sides must be lists. A string on the right is refused outright.

Read the message in two parts: can only concatenate list (the left side is a list, so the right must be one too) not "str" to list (what you actually supplied).

Cause 1 — you meant append, not concatenate

This is the cause in most reports. The intent was 'add this one item to the list', but + is not how you add an item:

names = ["Asha", "Ravi"]
new = "Meena"

names = names + new        # TypeError

Traceback (most recent call last):
  File "demo.py", line 4, in <module>
    names = names + new
            ~~~~~~^~~~~
TypeError: can only concatenate list (not "str") to list

Three correct shapes, depending on what you want:

names.append("Meena")            # add ONE item in place -> ['Asha','Ravi','Meena']
names += ["Meena"]               # same thing, wrapped in a list
names = names + ["Meena"]        # new list, original untouched

names.extend(["Jo", "Kim"])      # add SEVERAL items in place

Cause 2 — you meant to build a string

Sometimes the list was never meant to be a list. If the goal is text, convert explicitly or use an f-string:

items = ["milk", "eggs"]

print("Shopping: " + str(items))       # Shopping: ['milk', 'eggs']
print(f"Shopping: {items}")            # same, and clearer
print("Shopping: " + ", ".join(items)) # Shopping: milk, eggs  <- usually what you want

Cause 3 — a function returned a list where you expected text

The same crash appears when a value's type is not what the surrounding code assumes — a function returning a list, a JSON array, or str.split():

def tags(post):
    return ["python", "debug"]     # a list, not a string

line = "Tags: " + tags({})          # TypeError
line = "Tags: " + ", ".join(tags({}))   # Tags: python, debug

The rule, and the sibling errors

+ in Python is type-strict and each type defines it differently. There is no automatic conversion between text and collections:

[1, 2] + [3]        # [1, 2, 3]     list + list  -> new list
[1, 2] + "3"        # TypeError     list + str   -> this error
"1" + "2"           # '12'          str + str    -> new string
"1" + 2             # TypeError     str + int    -> 'can only concatenate str (not "int") to str'
(1, 2) + (3,)       # (1, 2, 3)     tuple + tuple

When in doubt, print type(value) on both sides of the +. The mismatch is always visible in one line.

How to debug it

1. Ask what you actually wanted: one more item in the list, or one longer string? The fix follows from the answer.
2. For an item, use append (one) or extend (several) — not +.
3. For text, use an f-string or str(value); ", ".join(...) if the goal is readable output.
4. Print type() of both operands. If one came from a function call, check what that function returns on every path.
5. Remember + never mutates — names + [x] builds a new list and throws the old one away unless you assign it. append mutates and returns None.

Fix it interactively

These free problems are the same list-and-string mix-up — repair them in the browser:

• extend() vs append() easy

Related messages worth knowing

• can only concatenate str (not "int") to str — the same rule with the string on the left.
• unsupported operand type(s) for + — the third wording of the same type-strict rule.
• TypeError: the parent guide — the famous wordings decoded together.