LangChain Expression Language (LCEL) basics - Step-by-Step Execution

Concept Flow - LangChain Expression Language (LCEL) basics

Start: Define Expression

↓

Parse Expression

↓

Evaluate Variables

↓

Apply Operators

↓

Return Result

LCEL processes expressions by parsing, evaluating variables, applying operators, and returning the result.

Execution Sample

LangChain

expression = "{x} + {y}"
variables = {"x": 5, "y": 3}
result = lcel.evaluate(expression, variables)
print(result)

This code evaluates a simple addition expression using LCEL with variables x and y.

Execution Table

Step	Action	Expression State	Variables	Result
1	Start with expression	"{x} + {y}"	{"x": 5, "y": 3}	N/A
2	Parse expression	"{x} + {y}"	{"x": 5, "y": 3}	N/A
3	Evaluate variable x	"5 + {y}"	{"x": 5, "y": 3}	N/A
4	Evaluate variable y	"5 + 3"	{"x": 5, "y": 3}	N/A
5	Apply + operator	8	{"x": 5, "y": 3}	8
6	Return result	8	{"x": 5, "y": 3}	8

💡 Expression fully evaluated and result 8 returned.

Variable Tracker

Variable	Start	After Step 3	After Step 4	Final
x	undefined	5	5	5
y	undefined	undefined	3	3

Key Moments - 2 Insights

Why do variables get replaced step-by-step instead of all at once?

LCEL evaluates variables one by one as shown in steps 3 and 4 of the execution_table to ensure correct order and handle dependencies.

What happens if a variable is missing in the variables dictionary?

If a variable is missing, LCEL cannot replace it, causing an error or incomplete evaluation, as variables must be defined before evaluation.

Visual Quiz - 3 Questions

Look at the execution_table, what is the expression state after step 4?

A "8"

B "{x} + 3"

C "5 + 3"

D "{x} + {y}"

💡 Hint

Check the 'Expression State' column at step 4 in the execution_table.

At which step does the operator get applied to produce the final result?

A Step 3

B Step 5

C Step 2

D Step 6

💡 Hint

Look for the step where the '+' operator is applied in the execution_table.

If variable y was 10 instead of 3, what would be the final result?

A 15

B 13

C 5

D 8

💡 Hint

Add x=5 and y=10 as per the variable_tracker and execution_table logic.

Concept Snapshot

LCEL basics:
- Write expressions with variables in braces, e.g. "{x} + {y}"
- Provide variables as a dictionary
- LCEL replaces variables step-by-step
- Operators are applied after variable substitution
- Result is returned as evaluated value

Full Transcript

LangChain Expression Language (LCEL) lets you write expressions with variables inside braces like {x} and {y}. You provide a dictionary of variable values. LCEL parses the expression, replaces variables one by one, then applies operators like +. Finally, it returns the computed result. For example, "{x} + {y}" with x=5 and y=3 evaluates to 8. Variables must be defined before evaluation. This stepwise process ensures correct and clear evaluation.