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I have been trying to make a gate in qiskit in terms of the basic gates, but I keep on getting an error when I apply it to a circuit.

This is my gate:

class LGate(Gate):
 
    def __init__(self, label=None):
    
        super().__init__('l', 1, [], label=label)

    def _define(self):

  
        from qiskit.circuit.quantumcircuit import QuantumCircuit
        from qiskit.circuit.library import U3Gate
        q = QuantumRegister(1, 'q')
        qc = QuantumCircuit(q, name=self.name)
        rules = [
            (U3Gate(pi, 0, pi), [q[0]], [])
        ]
        qc._data = rules
        self.definition = qc

Of course this is just an X gate, but I was just trying a basic example.

Running the program:

circ = QuantumCircuit(2)
circ.l(0)
print(circ)

Error:

AttributeError: 'QuantumCircuit' object has no attribute 'l'
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You only defined the gate, but not you haven't added the according method to the circuit. Add the following to the QuantumCircuit class:

    def l(self, qubit): 
        # adjust this import to the location of your gate
        from .library.standard_gates.l import LGate
        return self.append(LGate(), [qubit], [])

See for instance here how it works for the SwapGate.

Just as a note: It's best practice to not use the letter l because that can be difficult to distinguish from an I in some fonts.

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I think you can also use the method of composite gates which might be easier to implement. The idea is that you create a circuit with gates and then turn it into an instruction by using the to_instruction() method. Once you've done this, you can consider this instruction as a predefined gate and add it to your new circuit by using the append() method. Let me show you an example code. I'll follow your example for X gate, but this code can be generalized to arbitrary gates.

from qiskit import QuantumCircuit

# customize a gate instruction (e.g., X gate)
qc = QuantumCircuit(1, name='X')
qc.x(0)
custom_gate = qc.to_instruction()

# append custom gate to a new circuit
new_circ = QuantumCircuit(2)
new_circ.append(custom_gate, [0])
print(new_circ)

     ┌───┐
q_0: ┤ X ├
     └───┘
q_1: ─────
    

As you can see, we first define single-quibit circuit qc with a X gate and turn it into an instruction. Then it can be appended to any new circuit. Here we attach it to a two-qubit circuit with name new_circ.

I hope my answer would help.

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