Template Interpolation

Pylee’s template interpolation system provides powerful, flexible string templating capabilities for dynamic configuration generation. This system supports variable substitution, secret handling, and context-aware processing across different environments and use cases.

Overview

Template interpolation in Pylee allows you to:

Dynamic value substitution: Replace placeholders with actual values at runtime
Variable and secret integration: Seamlessly incorporate variables and secrets
Context-aware processing: Different interpolation behavior based on usage context
Multi-level precedence: Resolve values from organization, registry, and server levels
Secure secret handling: Maintain security while enabling flexible configuration

Basic Syntax

Template Format

Template strings use curly braces {} to denote placeholders:

{
  "remoteUrl": "https://{HOST}:{PORT}/api/v1",
  "environment": {
    "API_ENDPOINT": "{PROTOCOL}://{HOST}/api",
    "DATABASE_URL": "postgresql://{DB_USER}:{DB_PASSWORD}@{DB_HOST}:5432/{DB_NAME}"
  }
}

Simple Variable Substitution

{
  "template": "Bearer {API_KEY}",
  "context": { "API_KEY": "sk-1234567890" },
  "result": "Bearer sk-1234567890"
}

Multiple Variables

{
  "template": "http://{HOST}:{PORT}/api",
  "context": { "HOST": "localhost", "PORT": "3000" },
  "result": "http://localhost:3000/api"
}

Variable and Secret Prefixes

Variables

Prefixed: {var.VARIABLE_NAME} - Explicitly references a variable
Unprefixed: {VARIABLE_NAME} - References a variable or secret (with secrets taking precedence)

Secrets

Prefixed: {secret.SECRET_NAME} - Explicitly references a secret
Unprefixed: {SECRET_NAME} - References a variable or secret (with secrets taking precedence)

Prefix Examples

{
  "templates": {
    "explicit_variable": "Database host: {var.DB_HOST}",
    "explicit_secret": "API key: {secret.API_KEY}",
    "unprefixed": "Token: {AUTH_TOKEN}",
    "mixed": "Connect to {var.HOST} with key {secret.API_KEY}"
  }
}

Precedence Resolution

Precedence Order

Variables and secrets are resolved in the following order of precedence:

Server level: Most specific configuration
Registry level: Shared configuration across servers
Organization level: Global organizational defaults

Precedence Example

{
  "organization": {
    "variables": { "API_URL": "https://org.api.com" },
    "secrets": { "MASTER_KEY": "org-secret" }
  },
  "registry": {
    "variables": { "API_URL": "https://registry.api.com" },
    "secrets": { "SERVICE_KEY": "registry-secret" }
  },
  "server": {
    "variables": { "DEBUG": "true" },
    "secrets": { "API_KEY": "server-secret" }
  }
}

Template {API_URL} resolves to "https://registry.api.com" (registry overrides organization).

Variable vs Secret Precedence

When both a variable and secret have the same name at the same level, secrets take precedence for unprefixed references:

{
  "configuration": {
    "variables": { "TOKEN": "var-token" },
    "secrets": { "TOKEN": "secret-token" }
  },
  "templates": {
    "unprefixed": "{TOKEN}",
    "explicit_var": "{var.TOKEN}",
    "explicit_secret": "{secret.TOKEN}"
  },
  "results": {
    "unprefixed": "secret-token",
    "explicit_var": "var-token",
    "explicit_secret": "secret-token"
  }
}

Advanced Features

Default Values

Provide fallback values when variables might be undefined:

{
  "templates": {
    "with_default": "Port: {PORT:-3000}",
    "complex_default": "Database: {DB_URL:-postgresql://localhost:5432/default}"
  }
}

Nested Interpolation

Variables can reference other variables:

{
  "variables": {
    "PROJECT_ROOT": "/opt/myapp",
    "DATA_DIR": "{PROJECT_ROOT}/data",
    "LOGS_DIR": "{PROJECT_ROOT}/logs"
  },
  "templates": {
    "data_path": "Data stored in {DATA_DIR}",
    "log_path": "Logs written to {LOGS_DIR}"
  }
}

Complex Template Patterns

{
  "templates": {
    "api_endpoint": "{PROTOCOL}://{HOST}:{PORT}/api/{VERSION}",
    "database_url": "postgresql://{DB_USER}:{secret.DB_PASSWORD}@{DB_HOST}:{DB_PORT}/{DB_NAME}",
    "auth_header": "Bearer {secret.API_KEY}",
    "service_config": "{var.SERVICE_NAME} running on {HOST}:{PORT} (debug: {var.DEBUG})"
  }
}

Context-Aware Processing

Display Context

Templates processed for UI display with secrets masked:

{
  "template": "API Key: {secret.API_KEY}",
  "display_result": "API Key: [redacted]"
}

Copy Context

Templates processed for copying with full values:

{
  "template": "export API_KEY={secret.API_KEY}",
  "copy_result": "export API_KEY=sk-1234567890abcdef"
}

Execution Context

Templates processed for runtime execution:

{
  "template": "docker run -e API_KEY={secret.API_KEY} myapp",
  "execution_result": "docker run -e API_KEY=sk-1234567890abcdef myapp"
}

Usage Examples

Basic Configuration

{
  "server": {
    "variables": {
      "HOST": "localhost",
      "PORT": "3000",
      "PROTOCOL": "http"
    },
    "secrets": {
      "API_KEY": "sk-1234567890abcdef"
    }
  },
  "templates": {
    "endpoint": "{PROTOCOL}://{HOST}:{PORT}/api",
    "auth": "Bearer {secret.API_KEY}"
  },
  "results": {
    "endpoint": "http://localhost:3000/api",
    "auth": "Bearer sk-1234567890abcdef"
  }
}

Multi-Environment Configuration

{
  "environments": {
    "development": {
      "variables": {
        "HOST": "localhost",
        "PORT": "3000",
        "DEBUG": "true"
      }
    },
    "production": {
      "variables": {
        "HOST": "api.production.com",
        "PORT": "443",
        "DEBUG": "false"
      }
    }
  },
  "templates": {
    "api_url": "https://{HOST}:{PORT}/api",
    "debug_mode": "Debug enabled: {DEBUG}"
  }
}

Database Configuration

{
  "database": {
    "variables": {
      "DB_HOST": "localhost",
      "DB_PORT": "5432",
      "DB_NAME": "myapp"
    },
    "secrets": {
      "DB_USER": "admin",
      "DB_PASSWORD": "secret123"
    }
  },
  "templates": {
    "connection_string": "postgresql://{secret.DB_USER}:{secret.DB_PASSWORD}@{DB_HOST}:{DB_PORT}/{DB_NAME}",
    "host_info": "Database host: {DB_HOST}:{DB_PORT}"
  }
}

Service Configuration

{
  "service": {
    "variables": {
      "SERVICE_NAME": "user-service",
      "VERSION": "v1.2.3",
      "ENVIRONMENT": "production"
    },
    "secrets": {
      "JWT_SECRET": "jwt-secret-key",
      "DATABASE_URL": "postgresql://user:pass@host:5432/db"
    }
  },
  "templates": {
    "service_info": "{SERVICE_NAME} {VERSION} running in {ENVIRONMENT}",
    "jwt_config": "JWT secret: {secret.JWT_SECRET}",
    "database_config": "Database: {secret.DATABASE_URL}"
  }
}

Integration with Pylee Components

ConnectButton Integration

The ConnectButton component uses template interpolation for dynamic configuration:

{
  "connectButton": {
    "remote": {
      "url": "https://{HOST}:{PORT}/mcp",
      "headers": {
        "Authorization": "Bearer {secret.API_KEY}",
        "X-Service-Version": "{var.VERSION}"
      }
    },
    "package": {
      "name": "{var.PACKAGE_NAME}",
      "version": "{var.PACKAGE_VERSION}",
      "args": ["--host", "{HOST}", "--port", "{PORT}"]
    }
  }
}

Environment Variable Generation

{
  "environmentVariables": {
    "API_ENDPOINT": "{PROTOCOL}://{HOST}:{PORT}/api",
    "DATABASE_URL": "postgresql://{DB_USER}:{secret.DB_PASSWORD}@{DB_HOST}:5432/{DB_NAME}",
    "SERVICE_NAME": "{var.SERVICE_NAME}",
    "DEBUG_MODE": "{var.DEBUG}",
    "SECRET_KEY": "{secret.APP_SECRET}"
  }
}

Command Generation

{
  "commands": {
    "docker_run": "docker run -e API_KEY={secret.API_KEY} -e DEBUG={var.DEBUG} {var.IMAGE_NAME}",
    "npm_start": "npm start -- --host {HOST} --port {PORT}",
    "python_app": "python app.py --config {var.CONFIG_FILE} --env {var.ENVIRONMENT}"
  }
}

Error Handling

Missing Variables

If a template variable is not found, the placeholder is left unchanged:

{
  "template": "Bearer {UNKNOWN_KEY}",
  "result": "Bearer {UNKNOWN_KEY}"
}

Malformed Templates

Malformed templates are left unchanged:

{
  "templates": {
    "malformed1": "{{VARIABLE}",
    "malformed2": "{VARIABLE}}",
    "malformed3": "{{VARIABLE}}"
  },
  "results": {
    "malformed1": "{{VARIABLE}",
    "malformed2": "{VARIABLE}}",
    "malformed3": "{{VARIABLE}}"
  }
}

Non-String Values

Non-string values are converted to strings:

{
  "variables": {
    "PORT": 3000,
    "DEBUG": true,
    "VERSION": 1.2
  },
  "templates": {
    "port": "Port: {PORT}",
    "debug": "Debug: {DEBUG}",
    "version": "Version: {VERSION}"
  },
  "results": {
    "port": "Port: 3000",
    "debug": "Debug: true",
    "version": "Version: 1.2"
  }
}

Performance Considerations

Template Compilation

Templates are compiled once and cached
Use template caching for frequently used patterns
Avoid complex nested interpolation chains

Memory Management

Clean up unused template contexts
Use weak references for large template sets
Monitor memory usage with many templates

Optimization Strategies

{
  "optimization": {
    "caching": {
      "enabled": true,
      "maxCacheSize": 1000,
      "ttl": 3600
    },
    "compilation": {
      "precompile": true,
      "validateOnCompile": true
    }
  }
}

Security Considerations

Secret Exposure Prevention

Never log interpolated templates containing secrets
Use context-appropriate secret handling
Implement proper access controls

Template Injection Prevention

Validate template syntax before processing
Sanitize user-provided template strings
Limit template complexity to prevent DoS

Secure Practices

{
  "security": {
    "templateValidation": {
      "enabled": true,
      "allowedPrefixes": ["var", "secret"],
      "maxTemplateLength": 1000,
      "maxNestingDepth": 5
    },
    "secretHandling": {
      "maskInLogs": true,
      "contextAware": true,
      "auditAccess": true
    }
  }
}

Testing Templates

Unit Testing

{
  "tests": [
    {
      "name": "basic_interpolation",
      "template": "Hello {NAME}",
      "context": { "NAME": "World" },
      "expected": "Hello World"
    },
    {
      "name": "secret_handling",
      "template": "API Key: {secret.API_KEY}",
      "context": { "secret.API_KEY": "sk-123" },
      "expected": "API Key: sk-123"
    },
    {
      "name": "precedence_test",
      "template": "{VALUE}",
      "context": { 
        "VALUE": "org-value",
        "secret.VALUE": "secret-value" 
      },
      "expected": "secret-value"
    }
  ]
}

Integration Testing

{
  "integrationTests": {
    "connectButton": {
      "template": "https://{HOST}/api?key={secret.API_KEY}",
      "contexts": ["display", "copy", "execution"],
      "expectations": {
        "display": "https://localhost/api?key=[redacted]",
        "copy": "https://localhost/api?key=sk-123",
        "execution": "https://localhost/api?key=sk-123"
      }
    }
  }
}

Migration Guide

From Legacy Template System

When migrating from older template systems:

{
  "migration": {
    "oldFormat": "${{ secrets.API_KEY }}",
    "newFormat": "{secret.API_KEY}",
    "steps": [
      "Identify all template strings",
      "Convert syntax to new format",
      "Update variable/secret references",
      "Test in all contexts",
      "Validate security handling"
    ]
  }
}

Batch Migration Tool

{
  "migrationTool": {
    "patterns": [
      {
        "from": "\\$\\{\\{\\s*secrets\\.([^}]+)\\s*\\}\\}",
        "to": "{secret.$1}"
      },
      {
        "from": "\\$\\{\\{\\s*vars\\.([^}]+)\\s*\\}\\}",
        "to": "{var.$1}"
      },
      {
        "from": "\\$\\{\\{\\s*([^}]+)\\s*\\}\\}",
        "to": "{$1}"
      }
    ]
  }
}

Troubleshooting

Common Issues

Template Not Resolving

Template: "Bearer {API_KEY}"
Result: "Bearer {API_KEY}"

Solutions:

Check variable exists in context
Verify correct prefix usage
Ensure proper precedence setup

Incorrect Precedence

Expected: "production-value"
Actual: "development-value"

Solutions:

Review precedence configuration
Check variable/secret levels
Verify context processing

Secret Exposure

Display shows: "Bearer sk-1234567890abcdef"
Expected: "Bearer [redacted]"

Solutions:

Check context configuration
Verify secret handling rules
Review component implementation

Debugging Tools

{
  "debug": {
    "templateInterpolation": {
      "enabled": true,
      "logResolution": true,
      "logContext": true,
      "logPrecedence": true
    }
  }
}

Best Practices

Template Design

Use descriptive variable names
Keep templates readable and maintainable
Avoid overly complex nested interpolation
Document template usage and dependencies

Variable Management

Use consistent naming conventions
Group related variables logically
Implement proper validation
Document variable purposes

Security

Never expose secrets in display contexts
Use appropriate prefixes for clarity
Implement proper access controls
Regular security audits

Performance

Cache compiled templates
Minimize template complexity
Use efficient variable lookups
Monitor performance metrics

Next Steps

Learn about Configuration Management for advanced scenarios
Explore Variables for non-sensitive configuration
Check Secrets for sensitive data management
Review Server configuration for complete examples

Get Started

Using Pylee

Understanding Pylee

MCP Servers

Support

​Template Interpolation

​Overview

​Basic Syntax

​Template Format

​Simple Variable Substitution

​Multiple Variables

​Variable and Secret Prefixes

​Variables

​Secrets

​Prefix Examples

​Precedence Resolution

​Precedence Order

​Precedence Example

​Variable vs Secret Precedence

​Advanced Features

​Default Values

​Nested Interpolation

​Complex Template Patterns

​Context-Aware Processing

​Display Context

​Copy Context

​Execution Context

​Usage Examples

​Basic Configuration

​Multi-Environment Configuration

​Database Configuration

​Service Configuration

​Integration with Pylee Components

​ConnectButton Integration

​Environment Variable Generation

​Command Generation

​Error Handling

​Missing Variables

​Malformed Templates

​Non-String Values

​Performance Considerations

​Template Compilation

​Memory Management

​Optimization Strategies

​Security Considerations

​Secret Exposure Prevention

​Template Injection Prevention

​Secure Practices

​Testing Templates

​Unit Testing

​Integration Testing

​Migration Guide

​From Legacy Template System

​Batch Migration Tool

​Troubleshooting

​Common Issues

​Template Not Resolving

​Incorrect Precedence

​Secret Exposure

​Debugging Tools

​Best Practices

​Template Design

​Variable Management

​Security

​Performance

​Next Steps

Template Interpolation

Overview

Basic Syntax

Template Format

Simple Variable Substitution

Multiple Variables

Variable and Secret Prefixes

Variables

Secrets

Prefix Examples

Precedence Resolution

Precedence Order

Precedence Example

Variable vs Secret Precedence

Advanced Features

Default Values

Nested Interpolation

Complex Template Patterns

Context-Aware Processing

Display Context

Copy Context

Execution Context

Usage Examples

Basic Configuration

Multi-Environment Configuration

Database Configuration

Service Configuration

Integration with Pylee Components

ConnectButton Integration

Environment Variable Generation

Command Generation

Error Handling

Missing Variables

Malformed Templates

Non-String Values

Performance Considerations

Template Compilation

Memory Management

Optimization Strategies

Security Considerations

Secret Exposure Prevention

Template Injection Prevention

Secure Practices

Testing Templates

Unit Testing

Integration Testing

Migration Guide

From Legacy Template System

Batch Migration Tool

Troubleshooting

Common Issues

Template Not Resolving

Incorrect Precedence

Secret Exposure

Debugging Tools

Best Practices

Template Design

Variable Management

Security

Performance

Next Steps