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

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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.

Practice

1. What is the main purpose of LangChain Expression Language (LCEL)?

easy

A. To create full applications without coding

B. To write small expressions that control LangChain chains

C. To replace Python in LangChain completely

D. To design user interfaces for LangChain

Solution

1. Step 1: Understand LCEL's role

   LCEL is designed to write expressions that help control how LangChain chains behave.

2. Step 2: Compare options

   Only To write small expressions that control LangChain chains correctly describes LCEL's purpose; others describe unrelated tasks.

3. Final Answer:

   To write small expressions that control LangChain chains -> Option B

4. Quick Check:

   LCEL purpose = control chains with expressions [OK]

Hint: LCEL is for small expressions controlling chains [OK]

Common Mistakes:

• Thinking LCEL replaces full programming
• Confusing LCEL with UI design tools
• Assuming LCEL builds entire apps

2. Which of the following is the correct syntax to use a variable named input in an LCEL expression?

easy

A. {{input}}

B. {input}

C. $input

D. input

Solution

1. Step 1: Recall LCEL variable usage

   In LCEL, variables are used directly by their name without extra symbols.

2. Step 2: Evaluate options

   Only input uses the variable name plainly, which is correct syntax in LCEL.

3. Final Answer:

   input -> Option D

4. Quick Check:

   Variable usage = plain name [OK]

Hint: Use variable names directly without braces or symbols [OK]

Common Mistakes:

• Adding braces or dollar signs around variables
• Using double curly braces like in templates
• Confusing LCEL with other templating syntaxes

3. Given the LCEL expression input + ' world' where input is 'Hello', what is the output?

medium

A. 'input world'

B. 'Hello+ world'

C. 'Hello world'

D. Error: invalid operation

Solution

1. Step 1: Understand the expression

   The expression adds the string ' world' to the variable input which holds 'Hello'.

2. Step 2: Perform string concatenation

   Concatenating 'Hello' + ' world' results in 'Hello world'.

3. Final Answer:

   'Hello world' -> Option C

4. Quick Check:

   String concat = 'Hello world' [OK]

Hint: Adding strings concatenates them in LCEL [OK]

Common Mistakes:

• Treating + as a literal character
• Expecting variable name output instead of value
• Assuming syntax error on string addition

4. What is wrong with this LCEL expression: if input == 'yes' then 'ok' else 'no'?

medium

A. LCEL does not support if-then-else syntax like this

B. The variable name input is invalid

C. The strings should use double quotes instead of single quotes

D. The expression is correct and will work

Solution

1. Step 1: Check LCEL conditional syntax

   LCEL uses ternary-like syntax: condition ? true_value : false_value, not if-then-else.

2. Step 2: Analyze the given expression

   The expression uses if-then-else which is not valid in LCEL, causing a syntax error.

3. Final Answer:

   LCEL does not support if-then-else syntax like this -> Option A

4. Quick Check:

   Conditional syntax = ternary only [OK]

Hint: Use ternary ?: for conditions, not if-then-else [OK]

Common Mistakes:

• Using if-then-else like in other languages
• Assuming single quotes cause errors
• Thinking variable names are restricted

5. How would you write an LCEL expression to return the length of a variable text only if it is not empty, otherwise return 0?

hard

A. text != '' ? length(text) : 0

B. length(text) if text else 0

C. text ? length(text) : 0

D. if text != '' then length(text) else 0

Solution

1. Step 1: Understand LCEL conditional syntax

   LCEL uses ternary syntax: condition ? true_value : false_value.

2. Step 2: Check condition for non-empty string

   To check if text is not empty, use text != ''.

3. Step 3: Combine condition and function

   Use text != '' ? length(text) : 0 to return length if not empty, else 0.

4. Final Answer:

   text != '' ? length(text) : 0 -> Option A

5. Quick Check:

   Conditional length check = text != '' ? length(text) : 0 [OK]

Hint: Use ternary with condition text != '' for non-empty check [OK]

Common Mistakes:

• Using if-then-else syntax
• Assuming text alone is false if empty
• Using Python-style if expressions