Container Types

Opshin has two main container types:

  • List[a]
  • Dict[k, v]

Note: The container types are generic, and the letter in square brackets ([]) are the type arguments


The Opshin List type is a container type that is used to hold multiple values of the same type. This is works just like the list type in Python.

listy: List[int] = [1, 2, 3, 4, 5]

Note: Opshin lists are actually linked-lists. This is because that's how lists are implemented in UPLC.

List Operations

You can add lists using the + operator.

print([1, 2, 3, 4] + [5, 6])
# prints "[1, 2, 3, 4, 5, 6]"

List Access

You may access elements at arbitrary positions within a list like this:

listy[3]  # will return the element at the 4th position (3rd with 0-based indexing), i.e. 4

If you want to count from the back, use negative numbers:

listy[-2]  # returns the second-to-last element, i.e. 4

List Slices

You may access slices of a list using the slicing syntax.

["a", "b", "c", "d"][1:3]  # returns ["b", "c"]

List Comprehension

Opshin supports Python's list comprehension syntax. This allows for very compact list initialization:

squares = [x**2 for x in listy]


The len method returns the length of the list as an int:

lenny: int = len(listy) 
lenny == 5  # True

Membership using in

You can check whether some element is included in a list of elements using the keyword in.

4 in [1, 2, 3, 4, 5]  # True
100 in range(10)  # False

Empty lists

Empty lists may only be created in annotated assignments. This is to ensure that the type of the empty list can be correctly inferred.

a = []  # Fails! Unclear what type this expression has!

a: List[int] = [] # This works!

Dict[k, v]

The Dict type represents a map from keys of type k to values of type v. It works just like the dict type in Python.

# A dictionary storing scores in a game.
scores: Dict[str, int] = {"god_binder": 12, "radio_knight": 42}

Dictionary Access

A dictionary implements a map. In order to find the mapped value of element x we can access it in the dictionary via dict[x].

scores["god_binder"]  # returns 12

Take care when accessing values that are not contained in the dictionary - in case of missing keys, the contract will fail immediately.

scores["peter_pan"]  # fails with "KeyError"

If you are not sure whether a key maps to something in the dictionary use dict.get(x, d). It will try to return the value mapped to by x in the dictionary. If x is not present it will return d.

It is important that d is of the value type v to guarantee type safety.

scores.get("god_binder", 0)  # returns 12
scores.get("peter_pan", 0)  # returns 0


The .keys() method returns a list of the keys in a dictionary,

players: List[str] = scores.keys() # ["god_binder", "radio_knight"]


The .values() method returns a list of all the values in a dictionary.

raw_scores: List[int] = scores.values() # [12, 42]


The .items() method returns a tuple of the each key-value pair in the dictionary. This is particularly useful if you want to iterate over all pairs contained in a dictionary.

for username, score in scores.items():
    print(f"{username} scored: {score}")
    # prints first "god_binder scored: 12" and then "radio_knight scored: 42"