String validation (min, max, regex) in FastAPI - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to set a minimum length of 3 for the username field.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    username: str = Field(..., min_length=[1])

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

A3

B5

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 or 1 which allows too short usernames.

Confusing min_length with max_length.

The min_length parameter sets the minimum number of characters allowed for the string. Here, 3 means the username must have at least 3 characters.

2fill in blank

medium

Complete the code to set a maximum length of 10 for the password field.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    password: str = Field(..., max_length=[1])

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

A8

B5

C12

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Setting max_length too low, making passwords too short.

Mixing up max_length with min_length.

The max_length parameter limits the string length to a maximum number of characters. Here, 10 means the password cannot be longer than 10 characters.

3fill in blank

hard

Fix the error in the regex pattern to only allow lowercase letters for the nickname field.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    nickname: str = Field(..., pattern=r"[1]")

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

A^[a-z]+$

B^[A-Z]+$

C^[a-zA-Z]+$

D^[0-9]+$

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase letters in the pattern.

Forgetting to anchor the pattern with ^ and $.

The regex ^[a-z]+$ means the string must contain only lowercase letters from start to end. Other options allow uppercase letters or digits, which are not correct here.

4fill in blank

hard

Fill both blanks to create a field that requires a string with minimum length 5 and matches only digits.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    code: str = Field(..., min_length=[1], pattern=r"[2]")

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

A5

B^[0-9]+$

C^[a-z]+$

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong regex that allows letters.

Setting min_length too low.

The min_length=5 ensures the string is at least 5 characters long. The regex ^[0-9]+$ restricts the string to digits only.

5fill in blank

hard

Fill all three blanks to create a field that requires an uppercase string between 2 and 6 characters.

FastAPI

from pydantic import BaseModel, Field

class User(BaseModel):
    tag: str = Field(..., min_length=[1], max_length=[2], pattern=r"[3]")

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

A2

B6

C^[A-Z]+$

D^[a-z]+$

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up min_length and max_length values.

Using lowercase regex instead of uppercase.

The field requires a string with at least 2 and at most 6 characters. The regex ^[A-Z]+$ ensures only uppercase letters are allowed.

Practice

1. What does the constr function in FastAPI's Pydantic models help you do?

easy

A. Validate string fields with rules like minimum length, maximum length, and regex patterns

B. Create integer fields with range validation

C. Automatically generate API documentation

D. Handle database connections

Solution

1. Step 1: Understand constr purpose

   constr is a Pydantic type used to validate string fields with constraints like length and regex.

2. Step 2: Compare options

   Only Validate string fields with rules like minimum length, maximum length, and regex patterns correctly describes constr usage for string validation.

3. Final Answer:

   Validate string fields with rules like minimum length, maximum length, and regex patterns -> Option A

4. Quick Check:

   constr = string validation [OK]

Hint: Remember constr is for string validation rules [OK]

Common Mistakes:

• Confusing constr with integer validation
• Thinking constr manages database or docs
• Assuming constr auto-generates API docs

2. Which of the following is the correct way to define a string field in a Pydantic model with minimum length 3 and maximum length 10?

easy

A. name: str(min_length=3, max_length=10)

B. name: constr(min=3, max=10)

C. name: constr(min_length=3, max_length=10)

D. name: str(min=3, max=10)

Solution

1. Step 1: Recall correct syntax for constr

   The correct parameters are min_length and max_length inside constr().

2. Step 2: Check options

   Only name: constr(min_length=3, max_length=10) uses constr(min_length=3, max_length=10) correctly. Options A, B, and D use wrong parameter names or types.

3. Final Answer:

   name: constr(min_length=3, max_length=10) -> Option C

4. Quick Check:

   Use min_length and max_length in constr [OK]

Hint: Use min_length and max_length, not min or max [OK]

Common Mistakes:

• Using min and max instead of min_length and max_length
• Trying to add length constraints directly to str type
• Mixing parameter names

3. Given this Pydantic model snippet:

from pydantic import BaseModel, constr

class User(BaseModel):
    username: constr(min_length=4, max_length=8, regex=r'^[a-zA-Z]+$')

What happens if you try to create User(username='abc123')?

medium

A. Validation error because username is too long

B. User instance created successfully

C. Validation error because username is too short

D. Validation error because username contains digits

Solution

1. Step 1: Analyze regex pattern

   The regex r'^[a-zA-Z]+$' allows only letters, no digits.

2. Step 2: Check input username

   Input 'abc123' contains digits '123', violating the regex.

3. Final Answer:

   Validation error because username contains digits -> Option D

4. Quick Check:

   Regex restricts digits, so error occurs [OK]

Hint: Regex restricts characters; digits cause validation error [OK]

Common Mistakes:

• Ignoring regex and only checking length
• Assuming digits are allowed by regex
• Confusing length errors with pattern errors

4. Identify the error in this FastAPI Pydantic model field:

from pydantic import BaseModel, constr

class Item(BaseModel):
    code: constr(min_length=5, max=10)

medium

A. code field must be an int, not constr

B. max should be max_length to specify maximum string length

C. constr cannot be used for string validation

D. min_length should be min to specify minimum length

Solution

1. Step 1: Check parameter names in constr

   The correct parameter for maximum length is max_length, not max.

2. Step 2: Identify error in code

   Using max=10 causes a syntax or validation error because it's invalid.

3. Final Answer:

   max should be max_length to specify maximum string length -> Option B

4. Quick Check:

   Use max_length, not max, in constr [OK]

Hint: Use max_length, not max, for string max length [OK]

Common Mistakes:

• Using max instead of max_length
• Confusing min_length with min
• Thinking constr is for int fields

5. You want to create a FastAPI model field password that must be between 8 and 16 characters and include at least one digit. Which is the correct way to define it?

hard

A. password: constr(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$')

B. password: constr(min_length=8, max_length=16, regex=r'^[a-zA-Z]+$')

C. password: constr(min=8, max=16, regex=r'^(?=.*\\d).+$')

D. password: str(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$')

Solution

1. Step 1: Check length parameters

   Use min_length=8 and max_length=16 in constr for length limits.

2. Step 2: Verify regex for digit inclusion

   The regex r'^(?=.*\\d).+$' ensures at least one digit is present.

3. Step 3: Validate option correctness

   password: constr(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$') correctly combines length and regex. password: constr(min_length=8, max_length=16, regex=r'^[a-zA-Z]+$')'s regex disallows digits. password: constr(min=8, max=16, regex=r'^(?=.*\\d).+$') uses invalid min and max. password: str(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$') wrongly applies regex to str type.

4. Final Answer:

   password: constr(min_length=8, max_length=16, regex=r'^(?=.*\\d).+$') -> Option A

5. Quick Check:

   Use constr with min_length, max_length, and digit regex [OK]

Hint: Use constr with min_length, max_length, and digit regex [OK]

Common Mistakes:

• Using min and max instead of min_length and max_length
• Using regex that excludes digits
• Trying to add regex to plain str type