LangChain Expression Language (LCEL) basics - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a simple LCEL chain that chains a prompt with an LLM.

LangChain

chain = [1] | llm

Drag options to blanks, or click blank then click option'

A"prompt"

Bx

Cadd

Dprompt

Attempts:

3 left

💡 Hint

Common Mistakes

Using a variable name like 'x' instead of 'prompt'.

Putting quotes around the variable name.

LCEL uses the pipe operator | to chain runnables. Assuming prompt and llm are defined, the chain is prompt | llm.

2fill in blank

medium

Complete the code to create an LCEL chain that includes an output parser.

LangChain

chain = prompt | llm | [1]

Drag options to blanks, or click blank then click option'

A"parser"

Bmultiply

Cparser

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using a number instead of a variable.

Putting quotes around the variable name.

A common LCEL chain is prompt | llm | parser, where parser is typically StrOutputParser().

3fill in blank

hard

Fix the code in the LCEL chain to correctly chain the prompt and llm using the pipe operator.

LangChain

chain = prompt [1] llm

Drag options to blanks, or click blank then click option'

A<=

B|

C==

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using mathematical comparison operators.

Using equality operator instead of the pipe.

LCEL chains runnables using the pipe operator |.

4fill in blank

hard

Fill both blanks to create an LCEL chain with prompt, llm, and parser using pipe operators.

LangChain

chain = [1] | llm [2] parser

Drag options to blanks, or click blank then click option'

Aprompt

B|

C<

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using equality or other operators instead of pipe.

Wrong variable name.

The full chain is prompt | llm | parser, using | operators and the prompt variable.

5fill in blank

hard

Fill all three blanks to create an LCEL chain: prompt piped to llm, piped to parser, with correct operators and names.

LangChain

[1] [2] llm [3] parser

Drag options to blanks, or click blank then click option'

Aprompt

B|

C==

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong operators like == or !=.

Incorrect variable names.

The chain is prompt | llm | parser.

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