Overview

File Input

As described in the input types page, the developer can choose FileInput when creating the app's inputs page, allowing the user to upload files whose data can be used within the app. When a file is imported, its data will be available in the Code step as a base64 string, which is composed of the file data preceded by the file type and the encoding type. For example, the resulting string of imported CSV and PNG files have the following respective formats:

"data:text/csv;base64,xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"
"data:image/png;base64,xxxxxxxxxxxxxxxxxxxxxxxxxxxxx"

where xxxxxxxxxxxxxxxxxxxxxxxxxxxxx is the base64 string encoding of the file data.

As an example, the File Import app imports a file and displays the the file type and the base64 encoded file data. Part of the Python code for this app is given below:

def main(inputs):
    [meta, data] = inputs['file'].split(";base64,")
    metadata = meta + ";base64,"

    return {
        "MetaData": metadata,
        "Base64 String": data
    }

The two keys MetaData and Base64 String are then used in the output page to display their values:

File Type: {{ outputs.MetaData }}
Base64 String: {{ outputs["Base64 String"] }}

Decoding File Data

Once the file data is uploaded, the developer can use the base64 Python library to decode and reconstruct a file-like object in order to manipulate the data. The base64.b64decode() function can be used to decode the data into a Bytes Object. As an example, the following code returns the file type, the base64 string representation of the data, and the decoded Bytes data.

import base64

def main(inputs):
    # Decode the file data
    [meta, data] = inputs['file'].split(";base64,")
    metadata = meta + ";base64,"
    decodeddata = base64.b64decode(data)
    return {
        "MetaData": metadata,
        "Base64 String": data,
        "decoded String": decodeddata
    }

File Output

In order to create and export a file for the user to download, the developer can use base64 encoding for data transfer. The function base64.b64encode() converts the Bytes data into base64 encoded bytes. Then, the .decode() method converts the base64 bytes into a string. The file type data should be added to the beginning of the encode data string and the resulting string can then be prepared for download using the <a> HTML tag. The href attribute of the <a> is to be set to the file data string, while the download attribute is set to the filename when the user attempts to download the file.

The following code of the Import File app uses data obtained from a file uploaded by the user. The file extension is obtained using the guess_extension from the mimetypes library. The code creates two file data that are then available for download. The first file is prepared using the original data, while the second file is prepared by first decoding the original data and then encoding it again. The extension of the second file in the download link is set to .txt.

from mimetypes import guess_extension, guess_type
import base64

def main(inputs):
    # Split the file data into two components meta and data. The split function removes the string ";base64",
    [meta, data] = inputs['file'].split(";base64,")

    # add ";base64" to meta
    metadata = meta + ";base64,"
    # recreate the original file data by concatenating metadata and data
    OriginalFile = metadata + data

    # create a byte_object by decoding the file data
    byte_object = base64.b64decode(data)
    # create a string of the decoded byte_object using the .decode() method.
    decoded_string_object = 0 # byte_object.decode()
    # using base64, the byte object can be encoded resulting in a byte object. The ".decode()" method returns the equivalent string.
    base64_converted_string = base64.b64encode(byte_object).decode()
    # creates a new file by concatenating metadata and the base64_converted_string
    NewFile = metadata + base64_converted_string

    # guess the extension from meta
    file_extension1 = guess_extension(guess_type(metadata)[0])
    # new file extension
    file_extension2 = ".txt"

    return {
        "MetaData": metadata,
        "Base64 String": data,
        "decoded Bytes": byte_object,
        "decoded String Object": decoded_string_object,
        "Download Original": "<a href='" + OriginalFile + "' download='Original" + file_extension1 + "'>Download</a>",
        "Download New": "<a href='" + NewFile + "' download='New" + file_extension2 + "'>Download</a>"
    }

Notice that the values of the Download Original and Download New are strings containing the <a> HTML tag, which has href set as the file data, and download set as the downloaded filename. When the user runs the app, the <a> tag will become a hyperlink that will download the file when clicked.

The output editor has the following text:

To download original file: {{ outputs["Download Original"] }}
To download new file (with extension .txt): {{ outputs["Download New"] }}

File Type: {{ outputs.MetaData }}
Base64 String: {{ outputs["Base64 String"] }}
Decoded Bytes Object: {{ outputs["decoded Bytes"] }}