
import matplotlib
import sys
def set_matplotlib_backend(backend):
    """
    Sets the specified matplotlib backend interactively.

    This function checks if 'matplotlib.pyplot' has already been imported. If it has, the backend cannot be changed and the function returns an error message. If not, the user can specify a backend to set for matplotlib. The backend must be one of the supported options listed in the details.

    Parameters:
    - backend (str): The name of the backend to set. It must be one of the predefined backends supported by matplotlib. A brief description of each backend and its uses is provided below.

    Available Backends and Their Uses:
    - 'GTK3Agg': Uses the Agg rendering in a GTK 3.x canvas (Python 3 only).
    - 'GTK3Cairo': Uses the Cairo renderer to draw on a GTK 3.x canvas (Python 3 only).
    - 'GTK4Agg': Uses the Agg rendering in a GTK 4.x canvas (Python 3 only, newer GTK version).
    - 'GTK4Cairo': Uses the Cairo renderer to draw on a GTK 4.x canvas (Python 3 only, newer GTK version).
    - 'MacOSX': Uses the native MacOSX backend.
    - 'nbAgg': Embeds the backend in Jupyter notebooks, allowing interactive features.
    - 'QtAgg': Uses the Agg rendering to draw on a Qt widget.
    - 'QtCairo': Uses the Cairo renderer to draw on a Qt widget.
    - 'Qt5Agg': Uses the Agg rendering to draw on a Qt5 widget.
    - 'Qt5Cairo': Uses the Cairo renderer to draw on a Qt5 widget.
    - 'TkAgg': Embeds Agg rendering in a Tkinter canvas.
    - 'TkCairo': Uses the Cairo renderer to draw on a Tkinter canvas.
    - 'WebAgg': Uses a web server and JavaScript (HTML5 Canvas) to render.
    - 'WX': Uses native WX drawing.
    - 'WXAgg': Embeds Agg rendering in a wxPython widget.
    - 'WXCairo': Uses the Cairo renderer to draw on a wxPython widget.
    - 'agg': Writes to a static image file using the Anti-Grain Geometry engine.
    - 'cairo': Writes to a static image file using the Cairo library.
    - 'pdf': Writes to a static PDF file.
    - 'pgf': Can be used to create figures for inclusion in TeX documents.
    - 'ps': Writes to a static PostScript file for use in printing and publishing.
    - 'svg': Writes to a static SVG file for use in web publishing or as vector graphics.
    - 'template': A null backend for testing purposes, not intended for direct use.

    Returns:
    - str: A message indicating whether the backend was successfully set or an error message if the setting failed or an invalid selection was made.

    Example Usage:
    ```python
    set_matplotlib_backend('TkAgg')
    ```
    """
    # First, check if matplotlib.pyplot has already been imported
    if "matplotlib.pyplot" in sys.modules:
        
        del sys.modules['matplotlib.pyplot']
    
    
    # Check the current backend
    current_backend = matplotlib.get_backend()
    if current_backend == backend:
        return f"Backend is already set to {backend}."
    
    # List available backends
    available_backends = matplotlib.rcsetup.all_backends
    if backend not in available_backends:
        return f"{backend} is not a recognized matplotlib backend. Available backends are: {', '.join(available_backends)}."

    try:
        # Use matplotlib.use() to set the backend
        matplotlib.use(backend, force=True)
        
        # Import pyplot to test if backend is set correctly
        import matplotlib.pyplot as plt
        plt.figure()
        print(f"Backend successfully set to {backend} and tested with a blank figure.")
        return f"Backend successfully set to {backend}."
    except Exception as e:
        return f"Failed to set backend to {backend}. Error: {e}"