         }
         
         contents = try FileManager.default.contentsOfDirectory(at: fullStoragePath, includingPropertiesForKeys: nil)
-        XCTAssert(!contents.isEmpty, "Prerequisite for tests fails. Expected dir to be empty \(String(describing: fullStoragePath))")
+        XCTAssert(!contents.isEmpty)
         
         try client.clearAllCache()
         
         contents = try FileManager.default.contentsOfDirectory(at: fullStoragePath, includingPropertiesForKeys: nil)
-        XCTAssert(contents.isEmpty, "Prerequisite for tests fails. Expected dir to be empty \(String(describing: fullStoragePath))")
+        XCTAssert(contents.isEmpty)
     }
     
     func testDeleteSingleFile() throws {

final class TUSClientTests: XCTestCase {
    
    var client: TUSClient!
    var tusDelegate: TUSMockDelegate!
    var relativeStoragePath: URL!
    var fullStoragePath: URL!

        try XCTAssertThrowsError(client.upload(data: data))
    }
    
    func testMultipleInstancesDontClashWithFilesIfPathsAreDifferent() throws {
        // Make multiple instances, they shouldn't interfere with each other's files.
        
        // Second instance
        let url = URL(string: "TUSTWO")!
        let configuration = URLSessionConfiguration.default
        configuration.protocolClasses = [MockURLProtocol.self]
        otherClient = TUSClient(config: TUSConfig(server: URL(string: "www.tus.io")!), sessionIdentifier: "TEST", storageDirectory: url, session: URLSession.init(configuration: configuration))
        
        // Prerequisites
        var contents = try FileManager.default.contentsOfDirectory(at: fullStoragePath, includingPropertiesForKeys: nil)
        XCTAssert(contents.isEmpty)

        let documentsDir = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)[0]
        let otherLocation = documentsDir.appendingPathComponent("TUSTWO")
        try? FileManager.default.removeItem(atPath: otherLocation.path)
        
        XCTAssertFalse(FileManager.default.fileExists(atPath: otherLocation.path, isDirectory: nil), "Prerequite failed, dir shouldn't exist yet in this test")

        for _ in 0..<6 {
            try client.upload(data: Data("abcdef".utf8))
            try otherClient.upload(data: Data("abcdef".utf8))
        }

        var otherClientContents = try FileManager.default.contentsOfDirectory(at: otherLocation, includingPropertiesForKeys: nil)
        XCTAssert(!otherClientContents.isEmpty)
        
        contents = try FileManager.default.contentsOfDirectory(at: fullStoragePath, includingPropertiesForKeys: nil)
        XCTAssert(!contents.isEmpty)

        otherClientContents = try FileManager.default.contentsOfDirectory(at: otherLocation, includingPropertiesForKeys: nil)
        XCTAssert(!otherClientContents.isEmpty)

        // Now clear cache of first client, second should be unaffected
        try client.clearAllCache()

        contents = try FileManager.default.contentsOfDirectory(at: otherLocation, includingPropertiesForKeys: nil)
        XCTAssert(!contents.isEmpty)

        otherClientContents = try FileManager.default.contentsOfDirectory(at: otherLocation, includingPropertiesForKeys: nil)
        XCTAssert(!otherClientContents.isEmpty)
    }
    
    func testIdsAreGivenAndReturnedWhenFinished() {
